I bought an Apple Watch but I didn’t like the strap color. Can I exchange only the straps?

If it is less than 14 days you can return it. Otherwise you will have to buy the strap you want from Apple’s collection or any of the dozens of 3rd party strap vendors.

If it is within 14 days of when you bought it.

We're all users on this forum. Take it to your Apple Store with your receipt and ask Apple.