For detecting texture colors under the player, look for "c_trace" and "hit" in the manual. TEX_COLOR can not be used for that.

The collision behavior you're getting with PH_CONVEX could be caused by a too-big hull of the other entity, or a skin width setting too large for the scale of your objects. At least that's the 2 things I'd suspect at first.

As to the hinge problem, does it stop suddenly after 2 turns, or does the movement slowly cme to a standstill?