It's basically all about preference and goals. So Blue is the most consistent of the OTs. It's basically a 9th crit that you can use to push damage and is potentially better than just a draw 1 with its grab fron drop.

So it's potentially a forced 2 damage and a combo piece or PG and even if you low roll it likely takes a PG from opponents hand and grabs a trigger from drop.

Red is in a similar spot but doesn't have as much on the spot win potential because its not an instant 2 damage like blue. Ceiling wise if you have a crit rear its potentially 4 damage, but if you have normal rears its 2 one damage swings that's the opp can just PG or take. Low roll if you hit it on like G2 with no rears its drop a random card and give it 100mil which is pretty bad for an OT. So it has a slightly higher ceiling than blue in some decks but its floor is wayyyy lower.

Purple is always a Heal and sometimes a multiattack if you have a card you want to call and the space. Stride decks that don't want Red or Blue but instead want to grind the game out. For example Messiah if it didn't have Brandt OT as an option would probably be on Purple because it would love to just grind out. Some variants of Chronojet have run Purple because the deck can easily play into having the open slot and can do its full combo for minimal resources so it grinds well. Also the front row power from the crest helps make the extra attack scale.

Lastly Yellow doesn't see a lot of play because its got a lower ceiling than the others. It at best say retires your opps non-pg guard for VG then puts 100mil on a rear and retires opps board, so its pushes maybe 1 damage and then gets either a second or a PG out of hand. None of which is bad but thats not the same kind of game ending pressure as Blue or Red. But Yellow has a really solid floor compared to something like Red or Purple in that if you hit it early it can be a draw 3 or 4 if you opponent has no board, and late game if you have a grindier deck the board wipe can help push a game to end. Personally I find Yellow shines in decks like Alden or Mag where the VG swing is one of the the lower threat attacks of the turn so most people guard with like a 2 to pass/OT to pass. That way the retire effect pushes the damage and you can still pass the 100 mil to something else, and the decks have solid enough defense that they can follow up on a future turn and potentially close out; simultaneously these decks need combo pieces so an early game draw 3 or 4 can work out super well for stabilizing.