There are some instances where combo colours make approach circles and fruits extremely hard to see due to background dim being enabled for the majority of the player base.
Some examples:
Combo Colours 5,6,7,8: beatmapsets/41686#osu/131564
Combo Colour 1: beatmapsets/670373#osu/1417797
Combo Colour 1: beatmapsets/248876#osu/635679
Combo Colour 8: beatmapsets/917159#osu/2193177
Combo Colour 3: beatmapsets/1091102#osu/2281078
Some sort of sentence under skinning could be put saying that dark colours (such as under Luminosity 60~80) is highly discouraged.
Edit: I'm aware this exists now already but under specific modes. I think generally is should be put under skinning in general ranking criteria instead of specificly modes.
Some examples:
Combo Colours 5,6,7,8: beatmapsets/41686#osu/131564
Combo Colour 1: beatmapsets/670373#osu/1417797
Combo Colour 1: beatmapsets/248876#osu/635679
Combo Colour 8: beatmapsets/917159#osu/2193177
Combo Colour 3: beatmapsets/1091102#osu/2281078
Edit: I'm aware this exists now already but under specific modes. I think generally is should be put under skinning in general ranking criteria instead of specificly modes.