If they are truly clear and colorless, the question is not what looks good on them, but what looks good on what's under them. Food "looking good" is about contrast of color and value - not just light and dark, but also opposites on the color wheel. If you use placemats, think of them as the plate - what colors go with your placemats? If you don't use placemats, what color is your tabletop and, hence, what colors go well with your tabletop?
Edit - Also consider texture. If the glass is smooth, foods with a more "rough" texture will look better (typically) than foods that are also smooth.
Some opposites on the color wheel: red and green, blue and orange, yellow and violet. Think along those lines as well as lightness and darkness, and you're good to go. (I won't get into things like split complimentary or triad colors, but you can look those up if you have the desire.)