It's impossible to diagnose a problem like this with a one-line description. If you post the image you are trying to draw and/or the code that is doing the drawing, it'll help.
I'm not aware of any situation that would cause the colors to change unless you are specifically passing a ColorMatrix to do so, but certain types of graphics may appear darker or degraded with slight scaling.
I think I had the same problem.
I used an ImageList, but somehow the images in the list were darker then they should have been.
I've removed the images and added them again, which solved the problem for me.
Not sure how I added the images in the first place... been a while.