Yeah, pretty much my opinion as well. That's why I saved those pictures some time after the last yearly storytime and patiently waited for a chance to ask others for opinions. Of course I almost forgot about it, but looking for Usagi memes can help you find interesting things.
Luth is good, old school and friends with Stan - he certainly can make color work, but there are many talents that would make colorized Usagi soar. Most of you mentioned shading. Luth uses pastel colors even for scenes happening at night, something you should expect in a massively cartoonish and stylized book like Asterix or Groo. While it doesn't look bad it also doesn't fully fit Usagi (who's cartoonish, but not a full cartoon). However, the biggest thing about Luth is his reliability. New talents come and go, but he's a tried veteran who never goes below a certain level of quality and delivers on time. Imagine how jarring a sudden change of the colorist would be. Still, it's crazy how a change in coloring style can change how you read some scenes.