I've seen fields that require green, or brown bb's. It would be nice for fields if they could get a "field only color". Just looking for options, paintball fields (most anyway) have a field paint only policy.
BB's are cheap. Sure customers will sneak in their own, and it would be near impossible to enforce. If you as a field owner stocks a good bio-degradable BB in a few different weights, I would think overtime your customers would just buy from you anyway. Field BB's only would be a win win in the long run, in my opinion at this point anyway...