Replace (swap) colour in Bitmap

Question

I have googled loads, and all I can find is changing the whole bitmap to a new colour.

The basis of it is that I want to swap all the Red in an image (RGB: 231,9,9. Hex E70909)
to lets say Fushia, or another RGB colour.

I can't seem to work out how to specifically swapp just a single colour for another.

The image is only 80x80px so I don't think GetPixel (which supposedly is slow) would impact it that much. That is, if GetPixel is the correct thing to use.