The only no-no when it comes to mixing and matching camo is that you don't want to have a super dark color over a super light color on your upper body. For example, I'd avoid wearing a light tan or AP long sleeve shirt under a MONBU t-shirt.... the darker color will sort of profile the human shape a bit. This only matters anyway if you are standing up, facing whatever is coming to you. By and large its moot. Other than that, I always mix and match, espcecially sitting on the ground. I wear camo with a bit more brown on the bottom (like the forrest floor) and something with a bit more grey/green on top (like an oak tree can be).

Hold still, wear gloves and a facemask (no exposed skin) and you'll be fine as long as you aren't wearing white, blue or blaze orange.