I am using photoshop CS2. for some reason in the picture im working on, i am only able to work in black, white and shades of grey. i choose color (green, say) for the forground color, and by the time i've said "ok" and gone back to the work space, its shown me a grey that is the same tone as the green i chose...but grey none the less. weird, right?
go to Image-Mode and make sure you're not in Greyscale mode. Switch to RGB and then you should be okay.
And make sure you aren't tageting a layer mask that can only use gray values.