This is a very difficult question to have a one size fits all answer. Color stacking agenst an opposite color titan definitely has advantages and disadvantages.
For the advantages of course you have big damage when you do match the color you need, in your example that would be purple. You also have elemental link that encourages color stacking.
The huge drawback is the randomness of the board. You are in trouble if you don’t draw enough purples to do anything. Now this can be negated somewhat by bringing battle items such as a tornado, hourglass, Mana pots, etc. But this can get quite expensive to do often. Also due to the random nature of the board I have noticed with the same exact team in the same exact titan, you may get 60k damage one attack and 9k another.
Another stratagy is to stack just one or two heros. For example if you are fighting a red titan. Green hero damage is weaker agenst this color and the titan will hit your green heroes harder. In this case some olauers will drop the green hero and take another blue. This alleviates the color I’m balance, while at the same time is not locking out 4 other colors if you where to stack a mono blue team. While it will be more difficult to get really high score it is unlikely to get really low scores in general.
Just remember the is no guarantee on anything when random variables are involved so you just have to kinda try and lower the probability of undesired outcomes.