Apple Watch series 1 lost color screen

How to Change the Grayscale Setting on an Apple Watch

1. Step 1: Open the Watch app on your iPhone.
2. Step 2: Touch the My Watch tab at the bottom of the screen.
3. Step 3: Choose the General option on this menu.
4. Step 4: Select Accessibility.
5. Step 5: Tap the button to the right of Grayscale to turn it off.

You might want to wipe it clean and start all over again.

What to do before you sell, give away, or trade in your Apple Watch, or buy one from someone else - Apple Support

Don't load the backup. It will be a minor PIA reconfiguring the watch but the data is safe (it's duplicated in the iPhone).

Set up and pair Apple Watch with iPhone - Apple Support