For each body piece with the same color as the surronding surface, you may add 1 to the maximum camouflage value (2 arms, 2 legs, 1 torso, 1 head, which means 6 points maximum), with a minimum of 3 (which means you can't be "stealthy" if only your head is camouflaged, even if it gives you 1 of CV, Camouflage Value).
Then, the average minifig will roll 1d6. If the value exceeds the CV, the trooper will see it. If it is exactly the same, roll again and, if you roll more than or the same as the CV, the trooper sees the stealth unit.
It's easy. I use it a lot, actually.
Then there's blend-camo, which mimics a CV of 6, but costs 10CP (for those who use it).