In my experience, dubbels are often fairly dark, brown ales, while tripels are much clearer and lighter (of colour).
Why is that?
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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.