When I try to fill solid color or a gradient in selected area (Photoshop) the colors appears to be gray only.?

I am a learner of photoshop & using CS3. While trying to know the features of different buttons I clicked on something & that changed color setting. Then onward whenever I tried to fill solid color or a gradient, the color that appears in the selected area is a shade of GRAY & not the desired color. What is that adjustment I made by mistake ? How can I readjust the things so that I can fill the desired color?