Green = extremely easy , mom dad and the kids on a tag along
Blue = easy, smooth surface though some more recent ones can be nailed at full speed, with small berms and rollers. no drops, roots or rocks
red= moderate, everything can be rolled at slow speed but at high speed you can get some air with table tops, jumps and drops. expect some roots and rocks mid trail. medium length (up to 15 to 20k)
Black = hard to extreme, bigger drops, rocks roots. more of a natural feel. some are very long and demanding (glentress), some are very technical (wolftrax) where trials skills come in handy.
Orange = can generally be rolled at slow speed but are designed with big berms , jumps and table tops.
The thing about trail centers is that on any given trail colour once you have ridden a section of it you will know what level of skill you need. You are not going to come across anything more difficult on that trail. Whereas Natural trails will throw all sorts of obstacles at you so you need to be more on your game.