I think the best way to do what you want is by converting the image to an image with palette. This of course means changing the bits from 24 to 1-8 bits.
I believe there are some links on the internet on how to do this. I did a quick search and found the following:
MSDN convert image
C# RGB to Palette Based 8-bit Greyscale Bitmap Class
Converting 24 bit image
I also found the following demo that lets you convert the bits of the image while keeping the image colored:
Silverlight image processing
Hope this helps