public void setAlpha(int alpha) {
        alpha += alpha >> 7;   // make it 0..256
        int baseAlpha = mState.mBaseColor >>> 24;
        int useAlpha = baseAlpha * alpha >> 8;
        int oldUseColor = mState.mUseColor;
        mState.mUseColor = (mState.mBaseColor << 8 >>> 8) | (useAlpha << 24);
        if (oldUseColor != mState.mUseColor) {
            invalidateSelf();
        }
    }
Sets the color's alpha value.

@param alpha The alpha value to set, between 0 and 255.
menudrawer/src/net/simonvt/menudrawer/ColorDrawable.java