Have you looked at how you’re fixed for heroes against the class trials? The excellent spreadsheet by Kamikaze Assassin has a tab where you can put your roster in and check. That may inform some of your decisions. The spreadsheet is in the middle of this post:
Absent that analysis…
For your purple, I love my Proteus. He is amazing. Sabina is nice tile damage, decent heals, and an ability that duplicates Melendor. Proteus and Sabina are both pretty squishy, though, and would really benefit from going all the way up. Proteus is undoubtedly more useful against titans.
For your yellow, it’s hard to argue AM for Li when you have Drake sitting right there. She’s pretty sturdy at 3/60, and honestly her damage never gets brilliant.
For your blue, Kiril and then Sonya looks right. You’re light on maxed healers, but the one you do have leveled duplicates Sonya’s buff removal.
For green, who’s your tank right now? I know you said titains>raids, but Kashh is an amazing tank, and can hold cups ok up through low Diamond. Tanks benefit from the extra stats quite a lot, so I might consider him over Caedmon right now. If your heart is set on titans, then definitely Caedmon is nice. He’s got decent defense and decent tile damage.
For red, it’s honestly a matter of personal taste. Colen hits like a truck but is slow. Scarlett hits more lightly, but faster and with a debuff. Which do you find yourself using more? Colen’s AOE is honestly less useful than Scarlett’s debuff on titans. She’ll save you healing and mana pots over time.