Hi Diane - Yes, Dyzl is right, if you are having trouble with the color of the day, just play your best team. But it's up to you, we want everyone to have fun first.
I tend to build teams as follows, First position almost always has skull mitigation. Armored, Stoneskin or Gorgotha. Some good troops you probably have to try there would be Knight Coronet, Golem and Treant. But just search Stoneskin. For an armored troop that works really well as a tank, try Morthani's Will, he is armored and steals more armor.
Then I usually have two damage troops, one that is single targeted, like Lady Sapphira. The other should be someone that can damage all troops. Crimson Bat, Atlanta, many hero weapons, Psion.
The last troop will be situational, but often a debuff/mana drain choice, Dracos, Manticore, Spirit Fox, Succubus all are good.
The part I left out is mana acceleration, if I have a really good true damage AOE like Crimson Bat, I will sometimes give something up for mana acceleration. But since I run with Gorgotha up front a lot, I don't usually need it, as I think he is one of the best accelerators in the game.
Hope this helps, ask any questions you have.