The full length only comes in black or white. You're correct. But you can get crew length in over 10 different colors.
What you do is put either length you choose on with your shin guards. Then you take your team color sock..cut them at the ankle..slide those on over..tape the slipped on team sock at the ankle. Bam!! Done deal. Lots of pros do it this way. I think they might even show that on the website but I didn't look.
Another believer of Trusox here. My dd was getting what she called, "inception blisters", which were basically blisters on top of blisters. Once we went with Trusox, they went away. She's been doing the cutting or "Ronaldo" method of wearing her socks and hasn't had any problems in games. At $40 bucks a pop, it can get expensive, but we wouldn't have it any other way. We have the ankle socks, the mid-calf and the full size. Unfortunately, the full-size only comes in the cushioned thickness, which she wasn't a fan of, but it all works.