My puppy is very small with short legs. He is all brown with white paws, black ear tips, black face, and black tail tip. The man we bought him from had a Jack Russel Terrier and said it was the father and that the mom is also a JRT but didn’t have her with him.
Yes, JR’ can be combination of colors with not percentage of certain color.
Also there are 2 basic appearances of JR, the short-legged "Parson’s" line, used in Britain for tunnelling out ferrets, vermin etc. and the now longer legged JRs which breeders are now having, trying to breed out the shorter legged orig. JRs.
Hope this help.