Hi Bibbietje
A warm welcome to this great forum.
Out of curtesy, it is advised to issue a presentation post (your first name, scale, technique, your target project, your town / region, etc) This is not a police form to fill but rather to get a conversation started.

Back to your question:
- many item have fading colours after a while and many (like me) install a light filter to reduce ultra violet damaging rays
- in real life, different type of ballast are used (grey, brown, beige) depending on the place of extraction. Meaning, there are no real colour codes in ballast EXCEPT when integrating with ballast track (C-Track, etc) where the NOCH 09376 H0 gives better effects

If your question relates to grass imitation, "steumaterial" is an old technology to be replaced with fiber and electrostatic tool
It gives something like this



Important: do not expose your models (locos, houses) to sunlight over extended periods.
Cheers
Jean