Buff is used as a catch-all term in the UK for non-agouti rats with the red eye dilute gene, but the actual show standard for buff refers to a warm magnolia colour (think magnolia paint, rather than the magnolia flower) The eyes at first glance often look black, but they’re actually a very dark ruby colour..
Often buff rats can be too dark and with a grey or tan tone to them. In the US this is actually standardised as beige, with the paler buff shade we prefer in the UK being incorrect. It is possible for our buff to be too pale though, it isn’t a case of breeding for an extreme shade.
Genetics: red eyed black, red eyed chocolate, or red eyed mink (aarr, aabbrr, or aabbmm)
NFRS Standard: To be an even, warm magnolia with no sign of dullness or greyness. Belly colour to match top. Eyes to be dark ruby.