There are many different shades of chocolate, and there are many different shades of chocolate rat. A chocolate rat should be the colour of Dairy Milk, but they vary from Bournville-dark (and even almost impossible to distinguish from black), right the way down to milky Galaxy chocolate. As a dark self, the challenges with chocolates are similar to those of blacks: white toes, silvering, and intensity of colour.
It’s actually perfectly possible to breed a chocolate rat without the chocolate gene, but if you’re breeding chocolate in a line with something else (say, champagne or buff) then you’re best off seeking out true chocolates.
Genetics: Non-agouti with the chocolate gene (aabb), or sometimes non-agouti carrying mink and red eye dilute (aaMmRr). Not all blacks with that genetic combination will appear to be chocolate, but some of them will be. Other combinations of black carrying recessive genes can also appear as chocolate too.
NFRS standard: To be a deep, rich chocolate, as even as possible, devoid of any dinginess and white hairs or patches. Foot colour to match top. Eyes black.