(assuming you’re talking about the display colours and not the backlight) Pebble Time 2 uses a 64 colour e-paper memory LCD display. This page may help visualize which colours are available for developers to use in their apps and watchfaces: Color Picker Tool // Pebble Developers
Might help if you post a photo of the watch screen, too. Best colours will be in direct sunlight, depending on how blue your backlight is that might also affect things
You can see a small difference between white and yellow, but the yellow is so extremely dull, it’s almost no change in normal (daylight) light and none with backlight.
Only with the phone’s flashlight the difference is noticeable.
This might be a next rant about the dullness of the screen compared to my formerly used Time Steel. I did not suspect this; green, blue and red are acceptable, but especially yellow (a much used colour because it contrasts good like White, but should definitely be distinguishable from white)