Jess,
Whoever told you that it helps their color is blowing smoke...
It may with some animals, but not Cresteds.
As for them needing it, they don't. They're nocturnal lizards, so they hide when the lights are on.
I wouldn't do it, but if you do keep UVB on them, make sure not to oversupplement with Vitamin D3, which is easy to do when you're providing UVB. You could use a Calcium only supplement, but then you're going to have to worry about whether or not your cresteds are really absorbing the UVB (since they hide during the day).
In my opinion, there's no reason to provide UVB and there's a very good reason not to.
-Anthony
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Anthony Caponetto
www.ACreptiles.com