I believe on the X-Men 1 bonus disk Bryan Singer and Stan Lee addressed this topic. It has been like 6+ years when I watched it, but I believe Stan Lee (or someone else) said colorful costumes would not work. Comic books had colorful costumes because they are pictures are are supposed to be colorful. And Bryan Singer said he wanted the X-Men to look like a team with the same costume.
Thinking about it further, I guess it was his way with tying the X-Men with our present world (2000s). In our present world, black is the modern color. It is sleek and cool looking. However, in X-Men first class, they wore the classic blue and yellow X-Men Costumes. And at the end of that movie, Magneto had his classic Red and Pink Helm! Emma Frost was blooming white. And I thought all of that went really well and it was not distracting or hideous looking at all.
So maybe if X-Men 1 took place in the 60s, we would have seen more colorful costumes.
I always laugh at the joke Scott Summers says to Wolverine on the Jet: "What do you prefer, Yellow Spandex?". Looks like we will never get to see him wearing that.
That being said, I am curious if a Comic Book Movie with colorful costumes could work (I mean colorful, like every character having their own costume like Batman and Robin tried to do). Because, honestly, in X-Men 3: The Last Stand, even each character having their own little bright lining on their costumes (Kitty Pride having Pink, Iceman having blue, etc.) was too much color for me! I don't know why since it was so little color.
Also, don't forget though, the costumes in Batman & Robin were not only colorful, but some of the most poor quality costumes I have ever seen. They were basically Halloween Costumes. Poison Ivy looked like she bought her's at Wal-Mart. So maybe if movie makers actually put some good time, thought, and money into "colorful" costumes they may be able to make it work.