[Vectis Hamstery]

There's information about the yellow gene on the Midland Hamster Club site, as well as some info about other hamster genes.

It would depend what colour the tortoiseshell is and whether there are any shared recessives carried by both. Mating cream and yellow isn't a good idea as it muddies the waters and the cream can be very hard to get out of yellows later down the line. I wouldn't recommend that pairing regardless of the colour of the female.