The color of the watch band has changed, can it be exchanged for me?

Just to be sure I understand, which of the following are you stating:

1. The watch band's color has changed. That is, it has faded or worn off?
2. There is another watch band color available that you rather have then the one you currently have?

• For the former: The watch band comes with a one-year hardware warranty against manufacturing defects. As such, you would need to bring the band to your local Apple Store or Apple Authorized Service Provider to have them examine it to make that determination. This warranty does not cover "normal" wear & tear or abuse.
• For the latter: You have 14 days from the purchase date, to return the band for a replacement. After that, if you want a different color band, you will have to purchase it.

suppose you can try

Legal - Hardware Warranties - Apple

or reach out to their support channels

Get Support (apple.com)

And or

Contact - Official Apple Support

And or

Contact - How to Contact Us - Apple

Or

Genius Bar Reservation and Apple Support Options - Apple

If this is the band that came with your watch in the box, then it has the same 1 year warranty that came with your device. So if it's defective within that time frame, contact apple support for assistance: Contact Apple for support and service - Apple Support