I believe the main reason for tagging is to show the calves have been weaned rather than just taken from momma and put on a truck to the sale barn. Other colors would probably indicate weaning plus preconditioning (feed) or preconditioning plus shots....

A neighbor bought a couple calves straight "off the cow" as they say. These calves didn't seem to eat drink or do anything but wander the fenceline and bawl for quite some time - may have lost some valuable time to start to add weight.

Seems like the main thing as Roy mentions above is the weaning at least a couple weeks ahead of being sold. You can get the shots done but weaning damage/slower gain is there. Seems like a calf should have their shots before going to the sale barn also. Weaning seems like it can be a pain in the neck for the seller but valuable to the buyer? I am out of my element here but am trying "fenceline" weaning of a few late spring calves. 

I sat in at a sale barn in W SD a while back (had a good lunch/cafe) and they seemed to identify the farmer that raised them. The buyers also seemed to have more respect for and familiarity with certain sources.  Maybe knowing the source and how they prepare/precondition calves for sale is better than what color tag they may have...?

Jim at Dawn