Yes, he is definitely Colombian. I am just not sure if he is pastel or not. I'm sure he will be happy in his 55 gallon. Boas are not normally shy like ball pythons, and do well in larger cages. Make sure the lid is very secure, because they get really strong as they age. Be sure to post up some pics of his new cage once you get it all set up.
Rainbow boas are pretty neat looking. However, I have heard they are more of a challenge to care for than boas or ball pythons. I suggest doing a little research on their care before buying one.
Good luck on whatever you decide.