Rufousing is a polygenic trait which influences the “richness” or “warmth” of phaeomelanin pigment in agouti cats.
Polygenic means there isn’t just one “rufous gene,” but rather multiple genes which influence the phenotype. When it comes to rufousing in cats these genes have not yet been isolated and identified.
That hasn’t stopped breeders from being able to select for more or less rufousing, though! The ruddy Abyssinian being an excellent example.
On the left is the ruddy Abyssinian WCF Ch Lissana’s Fiesta and on the right is White Wolf Phaeton.
These are both black-based ticked tabbies.
[Source: Sparrow’s Garden]
In tabby cats the “true” color is portrayed by the stripes, which is why what’s frequently referred to as a “brown tabby” is actually black-based.
The rufousing polygenes work on the banding in between.
Here are some more examples!
One the left is a cool-toned Exotic Shorthair with low rufousing from Puro Glamour and on the right we have a very warm-toned Persian with high rufousing from Belcanto.
And then on the left we have a Siberian with low rufousing compared to the very warm-toned Siberian with high rufousing on the right. Both cats are from SarSibi.
In silver cats rufousing is referred to as tarnishing. On the left we have a Maine Coon without tarnishing (tarnish id undesirable in the breed) from Imperial Family and on the right we have an American Curl with tarnishing from ILmatar. This cat is not a tortoiseshell.