Watercolor paintings are generally matted and placed behind glass or acrylic. Watercolor paint tends to look light and airy and has a translucent quality. To maintain that look, avoid mats and
frames that are dark and heavy. This is just a general rule, every picture should tell you what colors will work best, sometimes it will not follow general rules.
Tip: Watercolors are original art. Make sure they are properly preserved by using appropriate conservation grade framing materials to protect their value and their longevity