I don't see any advantage to marking the positive.  If it is the positive 
to a rig, touching it ground won't do a thing, just no power to radio. 
The only positive wire that could shock or cause a spark is the actual 
wire coming out of the supply.  You are actually more likely to cause 
problems by touching the ground where it should not.  There is a reason 
Kenwood and others fuse both leads on their power cables.
73
Butch
WA0VJR
Node 3148
Wallace, ks.