In my experience, dubbels are often fairly dark, brown ales, while tripels are much clearer and lighter (of colour).
Why is that?
Color tends to come primarily from the malt bill used (darker roasted malts lending darker color to a beer). In the case of Belgian beers, a "candi" sugar (derived from beets) may used as an additive, and different styles use different types of candi sugars. For dubbels in particular, the candi is a darker variety, made with a (more) substantial dose of molasses, which would darken the beer further. A tripel uses a candi with less molasses.