If you're going to look at more than a very small number of pixels, you do not want to use GetPixel. It is very slow! See these CodeProject search results[^] for better ways to do this!

There are two ways depending if you need performance or not. Use GetPixel (not performant) or LockBits and and seek the byte (s) (can be calculated using Scan0 position plus y * stride + ( x * bits per pixel) assuming non indexed - for indexed this will give a reference in the pallet object which will hold the colour. Many examples of basic GDI+ lockbits scanning on the web.