I wouldn’t use a rainbow team for titans. I’d play with color stacking.
As for your choices, I’d go with:
Yellow: Mist (crowd control) - she’s fast, def down for specials, mana slow, hits 3 and prevents them from getting buffs.
Blue: Grimm - one of the best blue 4* in the game. Hard hitting with def down. Also, don’t sleep on Kiril, he’s a very versatile support hero.
Red: BT - one of the best red 4* in the game, heals and boosts your attacks to much higher damage.
Green: not sure, I’m not a fan of either. I won’t advise you to do what I do, but I would feed green feeders to Brienne or other 4* and wait for Caedmon, Brynhild, Melendor etc.
Purple: Burt or Sabina. I’d say Burt is more useful overall, but since you have a pulverizer in Grimm and no dispeller like Sonya or Caedmon, Sabina may serve you well. I wouldn’t max them just yet, though. If you land Rigard, I’d give those traps to him.
As for 5*, it’s not advised to develop 5* in the beginning, mainly because they take a lot more resources and maxed 4* are often better than unascended 5*. That said, look for 5* who are valued for their special effects rather than stats. If the value of the special is independent of their stats (like Isarnia’s def down), I would level her at some point after Grimm because she’ll be useful at lower levels. But the ones who are mainly valued for their attack values will not be used to their full potential.