I am working on leveling up a Sniper, and I want to know what stat my Color Crystals should increase. Out of the 4 Cartel ones (Endurance, Crit Rate, Power, and Expertise), I ruled out Expertise and Endurance. But, I don't know which to pick out of the last 2. Should I go with Power or Crit Rate (for a DPS Sniper/Gunslinger)?
Since 4.0, the below has changed — you always want Crit instead of Power. See http://www.swtor.com/community/showthread.php?t=844210
This completely depends on your current and desired stat distribution. Most people stick with Power crystals but there's no reason you can't use fewer Crit mods/enhancements with Crit crystals. This guide has some recommended stat levels (about 150 Crit, rest in Power). The current gear tiers (and most past ones) tend to give a ton of Crit so it often makes sense to take Power where you have the option.
Only hilts/barrels affect Weapon Damage so crystals, like other mods, purely add stats.