This picture is not available for download, and as such, is not to be used for anything other than character design/colour reference. If you find a way to save it, you are stealing and will be reported. Usable lineart can be found here.
How do you know? It only ever said that she was "Tabby-and-white," which could be grey tabby, brown tabby, orange tabby, or even purple tabby. There are a few cats listed like that where a person just has to guess at what colour they are. I picked grey tabby because of the "breeze" part of her name.
Well, I'm pretty sure she's brown, considering that cats like Brindleface are described as dappled gray within the books; however Swiftbreeze was only ever described as tabby-and-white. I don't think they'd have a character and never tell us her base coat colour, anyways. Besides, if you hadn't noticed, for leaving out her base pelt colour her description was pretty lengthy, you'd think they'd tell us whether she's brown, gray or ginger over her eye colour. That's just my opinion, though.
OHHHHHHH... Ya see... I never looked at the description in the book. I looked at the description on the internet. And ya no... I like grey better! Keep on truckin'!