fillrectangle

How to paint an area of a bitmap with 'transparent white'?

I want to replace all pixels in a rectangular region of a Bitmap with 'transparent white' - i.e., a=0, b=255, r=255, g=255. FillRectangle doesnt do this - given a transparent brush, the existing pixels are unchanged. Do I need to use SetPixel individually for each pixel in the rectangle? ...