I've never really gotten this, personally. I think D2 in particular is plenty colourful/clean. It comes from just having more outdoorsy areas, where D1 was all underground.
Nothing I've seen in videos of D3 seems really out of place compared to the Blood Moore or the desert (which was BRIGHT YELLOW).
The dark/dank atmosphere in D2 didn't really kick in until Act 3 to me. And that was one of the worst acts of D2 to me.
Hm, yeah, there are bright areas in D2, but overall, it FELT somewhat dark. It's hard do describe. Kinda claustrophoic, depressive. The desert in D2 probably like what Capcom tried to achieve with RE5 and wasn't able to.
Sure, there are a lots of things which come together to create such a feeling, but I don't get any of those feelings when looking at D3 beta videos. It's almost...charming. Like Torchlight, a bit Anime somewhat...
I really need to get in the beta to see for myself.
If it feels just like Torchlight, there is no need to buy it, I'll just loose interest after a short time. Path of Exile kinda was nice (beta), but it had other problems and the camera perspecitve was horrible.