Hi MartinBerntsson,
Thank you for posting in the Apple Support Communities. I understand you no longer have color on your Apple Watch, and are unable to correct the issue.
It sounds like Grayscale may have been enabled.
"To assist users for whom color might impair visibility, Apple Watch lets you enable grayscale onscreen. After you set the filter, the settings apply system-wide."
"You can turn on and manage these features on your Apple Watch and from the Apple Watch app on your iPhone. On your Apple Watch, go to Settings > General > Accessibility > Vision."
The above information was taken directly from the following resource.
Cheers!