That depends on what you would include in that list. If you only want to include band colours that were only ever available with the purchase of a specific Watch, I think there were only the Hermes Orange and the Nike Bone/Black one. Many more straps (especially Nike, but also Hermes) were originally only available with a Watch but later became available seperately. Now, if you want to include things like lugs and pins and buckles, there were a ton of bands with (real and painted) gold, rose gold and also ceramic hardware that were never available seperately.
Correction: some of the earliest Nike bands actually never became available seperately: Black/Cool Grey, Flat Silver/White and Flat Silver/Volt.