先来张效果图

主要是继承View的类，通过Paint的画笔类，对小圆点即选择的位置做区域限制，获取到当前选择区域的像素转换成rgb。

核心类ColorPickerView

public class ColorPickerView extends View   {

   private Context mContext;
   private Paint mRightPaint;            //画笔
   private int mHeight;                  //view高
   private int mWidth;                   //view宽
   private Bitmap mLeftBitmap;
   private Bitmap bitmapTemp;

   private Paint mBitmapPaint;//画笔
   private PointF mLeftSelectPoint;//坐标
   private OnColorBackListener onColorBackListener;
   private int mLeftBitmapRadius;
   public String colorStr="";
   private int initX = 0;
   private int initY = 0;
   private int r;//半径

   public ColorPickerView(Context context) {
      this(context, null);
   }

   public ColorPickerView(Context context, AttributeSet attrs) {
      super(context, attrs);
      mContext = context;
      init();
   }

   public void setOnColorBackListener(OnColorBackListener listener) {
      onColorBackListener = listener;
   }

   //初始化资源与画笔
   private void init() {
      bitmapTemp = BitmapFactory.decodeResource(getResources(), R.drawable.color_wheel);
      mRightPaint = new Paint();
      mRightPaint.setStyle(Paint.Style.FILL);
      mRightPaint.setStrokeWidth(1);
      mBitmapPaint = new Paint();

      mLeftBitmap = BitmapFactory.decodeResource(mContext.getResources(), R.drawable.color_wheel_dot);
      mLeftBitmapRadius = mLeftBitmap.getWidth() / 2;
      //获取圆心
      initX = bitmapTemp.getWidth() / 2;
      initY = bitmapTemp.getHeight() / 2;
      r = bitmapTemp.getHeight() / 2;
      mLeftSelectPoint = new PointF(0, 0);
   }

   //important patient please!!!
   @Override
   protected void onDraw(Canvas canvas) {
      canvas.drawBitmap(bitmapTemp , null , new Rect(0, 0, mWidth , mHeight ), mBitmapPaint);
      if(mLeftSelectPoint.x != 0 || mLeftSelectPoint.y != 0){
         canvas.drawBitmap(mLeftBitmap, mLeftSelectPoint.x - mLeftBitmapRadius,
               mLeftSelectPoint.y - mLeftBitmapRadius, mBitmapPaint);
      }
   }

   @Override
   protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
      mWidth = bitmapTemp.getWidth();
      mHeight = bitmapTemp.getHeight();
      setMeasuredDimension(mWidth, mHeight);
   }

   @Override
   public boolean onTouchEvent(MotionEvent event) {
      float x = event.getX();
      float y = event.getY();
      switch (event.getAction()) {
         case MotionEvent.ACTION_DOWN:
         case MotionEvent.ACTION_MOVE:
            proofLeft(x, y);
            invalidate();
            getRGB();
            break;
         case MotionEvent.ACTION_UP:
            //取色
            getRGB();
            invalidate();
      }
      return true;
   }


   private String toBrowserHexValue(int number) {
      StringBuilder builder = new StringBuilder(
            Integer.toHexString(number & 0xff));
      while (builder.length() < 2) {
         builder.append("0");
      }
      return builder.toString().toUpperCase();
   }

   /**
    * 像素转RGB
    */
   private void getRGB(){
      int pixel = bitmapTemp.getPixel((int)mLeftSelectPoint.x, (int)mLeftSelectPoint.y);
      int r = Color.red(pixel);
      int g = Color.green(pixel);
      int b = Color.blue(pixel);
      int a = Color.alpha(pixel);
      colorStr = "#" + toBrowserHexValue(r) + toBrowserHexValue(g)
            + toBrowserHexValue(b);    //十六进制的颜色字符串。
      if (onColorBackListener != null) {
         onColorBackListener.onColorBack(a, r, g, b);
      }
   }

   /**
    * 当view离开附着的窗口时触发，该方法和 onAttachedToWindow() 是相反
    */
   @Override
   protected void onDetachedFromWindow() {
      if (mLeftBitmap != null && mLeftBitmap.isRecycled() == false) {
         mLeftBitmap.recycle();//图片回收
      }
      if (bitmapTemp != null && bitmapTemp.isRecycled() == false) {
         bitmapTemp.recycle();//图片回收
      }
      super.onDetachedFromWindow();
   }

   // 校正xy
   private void proofLeft(float x, float y) {
      int r = bitmapTemp.getWidth() / 2 - mLeftBitmapRadius / 2;//圆半径
      //北
      PointF N = new PointF(initX,initY - r);//北
      PointF S = new PointF(initX,initY + r);//南
      PointF W = new PointF(initX - r,initY);//西
      PointF E = new PointF(initX + r,initY);//东
      int a = twoSpotGetLine(initX,initY,x,y);//圆心到点

      if(a < r){//在圆内
         mLeftSelectPoint.x = x;
         mLeftSelectPoint.y = y;
      }else{
         double angle = 0;
         int c = r;
         int b = 0;
         int newx = 0;
         int newy = 0;
         if(x > initX){//二四象限 NE SE
            if(y > initY){//四象限 东南ES
               b = twoSpotGetLine(S.x,S.y,x,y);//南点到点
               double aoccos = (Math.pow(a,2)+ Math.pow(c,2) - Math.pow(b,2)) / (2*a*c);
               angle = 90d - (Math.acos(aoccos)*(180/Math.PI));//角度
               Log.e("getLeftColor", "角度东南ES a: " + a + ",b: " + b + ",c: " + c + ",angle:" + angle);
               if(angle % 90 == 0){
                  newx = initX;
                  newy = initY + r;
               }
            }else if(y < initY){//二象限 北东EN
               b = twoSpotGetLine(E.x,E.y,x,y);//北点到点
               double aoccos = (Math.pow(a,2)+ Math.pow(c,2) - Math.pow(b,2)) / (2*a*c);
               angle = 360d - (Math.acos(aoccos)*(180/Math.PI));//角度
               Log.e("getLeftColor", "角度北东EN a: " + a + ",b: " + b + ",c: " + c + ",angle:" + angle);
               if(angle % 90 == 0){
                  newx = initX + r;
                  newy = initY;
               }
            }
         }else{//一三象限
            if(y > initY){//一象限 西北WN
               b = twoSpotGetLine(W.x,W.y,x,y);//西点到点
               double aoccos = (Math.pow(a,2)+ Math.pow(c,2) - Math.pow(b,2)) / (2*a*c);
               angle = 180d - (Math.acos(aoccos)*(180/Math.PI));//角度
               Log.e("getLeftColor", "角度西北WN a: " + a + ",b: " + b + ",c: " + c + ",angle:" + angle);
               if(angle % 90 == 0){
                  newx = initX - r;
                  newy = initY;
               }
            }else if(y < initY){//三象限 南西SW
               b = twoSpotGetLine(N.x,N.y,x,y);//东点到点
               double aoccos = (Math.pow(a,2)+ Math.pow(c,2) - Math.pow(b,2)) / (2*a*c);
               angle = 270d - (Math.acos(aoccos)*(180/Math.PI));//角度
               Log.e("getLeftColor", "角度南西SW a: " + a + ",b: " + b + ",c: " + c + ",angle:" + angle);
               if(angle % 90 == 0){
                  newx = initX;
                  newy = initY - r;
               }
            }
         }
         if(angle % 90 != 0){
            newx = (int)(initX + r * Math.cos(angle * Math.PI/180));
            newy = (int)(initY + r * Math.sin(angle * Math.PI/180));
         }
         Log.e("getLeftColor", "新坐标 x: " + newx + ",y: " + newy );
         mLeftSelectPoint.x = newx;
         mLeftSelectPoint.y = newy;
      }

//    图片区域
//    if (x < 0) {
//       mLeftSelectPoint.x = 0;
//    } else if (x > (LEFT_WIDTH)) {
//       mLeftSelectPoint.x = LEFT_WIDTH;
//    } else {
//       mLeftSelectPoint.x = x;
//    }
//
//    Log.e("proofLeft()", "proofLeft: " + x + "," + y);
//    if (x < 0) {
//       mLeftSelectPoint.x = 0;
//    } else if (x > (LEFT_WIDTH)) {
//       mLeftSelectPoint.x = LEFT_WIDTH;
//    } else {
//       mLeftSelectPoint.x = x;
//    }
//    if (y < 0) {
//       mLeftSelectPoint.y = 0;
//    } else if (y > (mHeight - 0)) {
//       mLeftSelectPoint.y = mHeight - 0;
//    } else {
//       mLeftSelectPoint.y = y;
//    }
   }

   /**
    * 两点取直线
    * @param x1
    * @param y1
    * @param x2
    * @param y2
    * @return
    */
   private int twoSpotGetLine(float x1,float y1,float x2,float y2){
      double line2 = Math.pow((x1-x2),2) + Math.pow((y1-y2),2);
      return (int)Math.abs(Math.sqrt(line2));
   }


   public String getColorStr() {
      return colorStr;
   }

   public void setColorStr(String colorStr) {
      this.colorStr = colorStr;
   }

   public interface OnColorBackListener {
      public void onColorBack(int a, int r, int g, int b);
   }
}

public class MainActivity extends Activity {

   private ColorPickerView colorDisk=null;
   private TextView tv;

   @Override
   protected void onCreate(Bundle savedInstanceState) {
      super.onCreate(savedInstanceState);
      setContentView(R.layout.activity_main);
      tv=(TextView)findViewById(R.id.tv_info);
      colorDisk=(ColorPickerView)findViewById(R.id.colorDisk);
      colorDisk.setOnColorBackListener(new ColorPickerView.OnColorBackListener() {
         @Override
         public void onColorBack(int a, int r, int g, int b) {
            tv.setText("R：" + r + "nG：" + g + "nB：" + b + "n" + colorDisk.getColorStr());
            tv.setTextColor(Color.argb(a, r, g, b));
         }
      });

   }

   @Override
   public boolean onCreateOptionsMenu(Menu menu) {
      getMenuInflater().inflate(R.menu.main, menu);
      return true;
   }

}

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    tools:context=".MainActivity" >

     <com.mrlin.mycolordisk.ColorPickerView
         android:id="@+id/colorDisk"
         android:layout_width="wrap_content"
         android:layout_height="wrap_content"
         android:layout_centerHorizontal="true"/>

     <TextView
         android:id="@+id/tv_info"
         android:layout_width="wrap_content"
         android:layout_height="wrap_content"
         android:layout_below="@+id/colorDisk"
         android:layout_centerHorizontal="true"
         android:layout_marginBottom="20dp"
         android:gravity="center_horizontal"
         android:layout_marginTop="15dp"
         android:text="颜色取值" />
     
</RelativeLayout>

最后附上源码

http://download.csdn.net/download/feiniyan4944/10200855