from pictures I've googled, it looks like pastels are a little brighter as adults, both showed some signes of browning, but with fires it was a little more obvious.
as for your caging question, are looking for a just one as a pet or looking to house a lot of snakes?
if the answer is just one or 2 as pets then the terrarium will be just fine, if you a looking to eventually have a lot of snakes, then a rack and tub system is best.
good luck in your search and I hope you can find the animal you are looking for to make you happy.
Cheers
That's a little backwards, fires get better and better with age, Idk if I'd call it yellow,but they are bright, pastels are known for Browning out, there are exceptions though.