As far as I know, there are no Gumblesh colors other than the default.
But hey, you never know what petpet colors TNT will do in the future, so go ahead and give it to your pet, if you like it. I've said time and again that a petpet doesn't have to match exactly; what matters most is if you're happy with it.
I'm tempted to do something similar with an Altachuck. I love the artwork, and its description is perfect for my Acara's personality, but there's just no color match yet. I'd like to see what other colors of Altachuck they come out with, especially Faerie, Pink, Purple, or Blue, before I switch out her petpet.
Or better yet, I wish they'd release that darn Faerie Magaral color. If they did, I'd keep loyal Sir Beebo right where he is now. Sure, it needs a little tweaking, but the basic idea's good...
"Alas...will my dream ever come true?" *sniffle*
"Meemeep meep meep, *tapdance* nyeep meep eep?"
(Roughly translated, it means "But aren't I cute enough just as I am?")