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.
In Dutch the names mean double and triple, they originated at the Belgian Westmalle abbey brewery where the double was a strong brown ale. Their tripel was the name they gave to their strongest beer, a blonde pale ale.
These beers quickly became popular and were imitated worldwide as were their names. The convention now is as following, a dubbel is a strong (6%-8% abv) dark ale, a tripel is a strong pale ale in the style of Westmalle tripel (9.5%). A quadrupel is an ale stronger than a tripel (+10%) but beyond that there is no real agreement on what it should be. Westvleteren 12 is an example of a quadrupel.