It's... there's lot more to it than I think People have really talked about.
For me, it comes down to a lot of factors, one is the move to digital colouring, it may not be for everyone, but there's certainly a lot more to play with there and it's a less tedious and cheaper than the old ways of colouring.
The most important part however is how it's seen, and that's the paper it's printed on/ tablet you are using.

I really, really, REALLY hate the paper Marvel uses, it's cheap, too easily destroyed and it limits the otherwise beautiful artwork on display. Compare that the paper DC is printed on, it's thicker but also the colours pop out more. The Demon Days One-shots are some of the best looking books I've seen from Marvel in a while, but the paper limits it too much. It spoils the look for me. If this was printed on the kind of paper DC uses, it'll pop out more.

A lot of the older books, mostly from 1960's-1990's had pretty rubbish paper, but it worked for the most part. It held together and the art work pops out, there's no interference when reading it.

Anyway, that's my war on crappy paper.