These two pets were color-changers for me too, when I first added them to my Pet Journal. It appears they've since lost that ability though. I've tried caging and relearning them, but nothing so far.
I've Tweeted at Jeremy Feasel and the official devs' Twitter to inquire about it, but I doubt I'll get any kind of direct response/answer.
Thanks very much for your kind help~
I've found something interesting about Young Venomfang this week.
I got another Young Venomfang from the supply in this new week. When I first added them to my Pet Journal, it was green.
My first Young Venomfang is blue and never be able to change its color now.
When I summon this new pet and it appears to be a color-changer.
I tried /reload for several times, it may even change its color in my Pet Journal. If it is blue in the Journal, it won't change its color; if it is green, it becoms a color-changer.
Today, I find a sad fact, it finally become blue and lost the ability to change colors forever.
IMO, it may be a bug.
In this case, Young Venomfang do have two skins, but we can only get the blue one at last......