I think that its the pythons (sorry no offence to the python lovers...I like pythons too but he asked me to choose...lol)
I think that the pythons just looks really cute when they're small. Once they get ollder their cute looks isn't on them anymore. I don't know...just not really my type of animals?
But if you say colorwise...maybe it's the colubrids that lose most of the bright colors. This is because there are a lot more choices (color wise) in colubrids. We simply haven't seen enough color variation in pythons and boas. Maybe if we come to the time when we see as many pythons and boas with as much colors as the colubrids, they too will lose their color too.
But this is just MHO