Sorry for the late reply and hopefully the information is still relevant. "Spanish Civil War Tanks" by Steve Zaloga (Osprey New Vanguard series) shows several photos of beret and leather clad tankers with the armor units. I'm sure leather jackets were great during the fall/winter engagements, especially in Northern Spain. Did show color shots of the vehicles but did not show color photos of the crews. The berets appear very dark and probably were black. Also, and not joking, remember the moustaches. Most photos show stylish tankers. :)
Jack