Like brightness, the contrast ratio of a projector is vital to the overall quality of the image. This ratio refers to the amount of difference between whites and blacks, so the higher the difference, the richer those colors (and the ones between them) will appear. You don't really want to go lower than a 1500:1 ratio for watching movies or anything similar, and anything higher will look great in most cases.
More affordable, portable projectors tend to fall below this, so it's up to you whether the other benefits outweigh the lack of color depth.