Hey Joe,
sometimes presets/preferences can get messed up or corrupted. It’s annoying but these things happen… 
Do you have any backups by any chance?
If not you should consider to do regular backups. Here are some options:
Backup & Restore function for Cubase on Windows
Backup function for Cubase on Mac
Profiles
Otherwise, do you have previous versions of Cubase on your system with colour preferences intact? In this case, try to rename your current C15 preference folder by adding a prefix like “_” to it while Cubase has not yet been launched. Afterwards, launch C15 and it will import previous settings from C14, C13.
Note: Some colour setups are saved with your project.
HTH