Generally, changing the sliders to Min/Max isn’t the best idea. The AI already has a in-built system of choosing what gems to match based off of what troops you are playing with, followed by what troops you are going against. It also takes into account troops which are currently at full mana, so it has a lower chance picking mana colors which are not needed.
The sliders are generally there to allow you to avoid picking skulls if you aren’t concerned with skulls, or if you have a mana generator which works on certain colors or if you want to avoid certain mana colors for your tank which you don’t care as much for. Using these sliders to form strategies of your own is what they are there for.