In order to fill in the gaps, you will need to do quite a bit of breeding. With John being an otter, you would need to breed him to a chocolate, something dilute, something REW or with REW to see what else is on the B_C_D_ locus.
I have had this grouping for a year, several litters from each, and still I cannot fill out the complete list of unknowns, even with pedigrees. I fill out what I can with successive breedings. I had a castor that produced a chocolate with the previous breeder, but with four successive breedings, even being bred back to her son, I never got a chocolate. That certainly tells me the others don't carry chocolate, but didn't prove she did.