Hi
For the Photos watch face, Apple Watch adjusts the colour of the time automatically, so that it can be read against whatever colour the background is in that area of the display.
It is not possible to customise the colour of the font manually.
If you would like to submit a feature request to Apple, then you can do so here:
Feedback - Apple Watch - Apple
The following resources explain which customisation features are available for each watch face, how to manage the watch face collection on your Apple Watch, how to customise supported features and how to configure which complications are displayed where on supported faces:
- Faces and features
Additional watch faces are available exclusively on Apple Watch Nike+ and Apple Watch Hermès; they cannot be installed on other models.
- Customize your watch face
- Change your watch face - Apple Support
- Use apps on your Apple Watch - Apple Support