Given that our default colours for enemy vs ally are red vs green and red/green colour-blindness is extremely common it could be a very nice feature to let people customise their highlight colours on the character settings page.
This is prompted by occasional fun on my side and others of mixing up red and purple but realising red/green colour-blindness is a thing makes me go it could be a nice broader accessibility fix if it isn't an absolute pain in the arse to code. No set of colours will be perfect for everyone so making them personal settings feels like the easiest way to deal with that, even if it's just type your own hex colours into the boxes for people to then work out some presets that they think work well.
Since currently we have:
Hostile
Enemy
Same Faction
No Relationship
Allied
Non-reciprocated friend
The current colours do a better job of highlighting allies and non-reciprocated friends as do not hit than factionmates.