To color BTAs:
Food, mega crazy PAR 600-1K, I'm sitting at about 700-800 on mine.
For food I feed tiny pellets soaked for 20 mins (fish, krill, shrimp all work great. Cyclopeez dipped shrimp would be fun to try) You want something that the nems can digest. the more the surface area the better.
Light, I run a ratio of "4 pure actinic, 4 Blue Special, and 1 warm white" 6 hours a day, drop to 2 actinic/2 blue special for 2 hour in the AM and PM.
To Bubble BTAs up:
High lighting does it every time, they will want to shade the oral disk. All of my BTA's bubble up, even the ones that had spindly tentacles for as long as I can remember. Usually takes a month or so.
If it is a mature nem (8"+), high lighting will usually force them to split so you'll want to increase lighting slowly. If you are feeding them regularly, they will be less inclined to wander around during light increases.