Calf boots are just a little taller than your average Dungeon Siege boot, and they come in a variety of solid colors:

Knee boots can only be worn in conjunction with lighter to medium armors (A1-A5 and A7); This is mostly a superficial restriction - heavy armors cover the legs with protective fittings which make it difficult to wear fitted high boots at the same time. If you equip a heavy body armor, the boots will be moved to inventory or dropped if there is no room.

Thigh boots take up 2x3 slots of inventory space each, and can only be worn in conjunction with lighter armors (A1-A4 and A7); This is mostly a superficial restriction - heavy armors cover the legs with protective fittings which make it difficult to wear fitted high boots at the same time. If you equip a heavy body armor, the boots will be moved to inventory or dropped if there is no room.

Variations:
This boot comes in 4 different varieties (first one to the left, and the three variation renders below).