Kittens are defined from when they are born to when they turn 1, so it would be the same for your cat.
Yes, a purebred British Shorthair can have a tabby coat.
Red is a possible color for the British Shorthair breed of cats but it is not a common color.
Yes, a purebred American Shorthair can have a silver tabby coat.
Yes, a purebred American shorthair can have a blue tabby coat.
Yes, a purebred British Shorthair can have a red tabby coat.
Tabby-like
yes, a purebred British Shorthair can have a blue tabby and white coat.
yes, a purebred American Shorthair can have a blue tabby and white coat.
Yes, a purebred American Shorthair can have a blue patched tabby coat.
Yes, a purebred American Shorthair can have a silver patched tabby coat.
Yes, a purebred British Shorthair can have a silver patched tabby coat.
Yes, the purebred British Shorthair can have a cream mackerel tabby coat.