final Drawable daWork = mBtnWork.getCompoundDrawables()[1]; // 1번째가 TOP 이미지, 이유는 위에서 android:drawableTop으로 하였기때문에..
if(daWork != null) mBtnWork.setCompoundDrawables(null, convertToGrayscale(daWork), null, null);

    protected Drawable convertToGrayscale(Drawable drawable)
    {
        ColorMatrix cmMatrix = new ColorMatrix();
        cmMatrix.setSaturation(0); //0이면 grayscale
        drawable.setColorFilter(new ColorMatrixColorFilter(cmMatrix));
        return drawable;
    }