Description

add rank colours so you can do /rankcolour so if you prefix was for example: [vip] you can change it to &6&l[&6&lvip&6&l] so it would be gold.

Proposed behaviour

It can change colours so the player can customize there rank how ever the like!

This is honestly a bad idea to add to LuckPerms and here is the reason:
LuckPerms itself does not whatsoever alter your chat or tablist. I get why people would think it does, but it doesn't.
The color codes you can use on prefixes and suffixes solely depend on the plugins that will display it, so for example chat or tablist - whichever plugin you may use. On 1.16 and above, if 'dispaly-plugins' support it, you can also use hex colors, but sometimes they take different coloring formats. Some plugins take '&6' for orange, others take '§6', the same goes for hex colors where the format it asks for itself really has no norm.
To implement your idea, luckperms would have to check what plugin a user is using as chat and tablist and have 'knowledge' of what the format is. Now also, since chat and tab are usually used separately, it can happen that they simply take different formats for colors, making this feature very confusing.
I like the idea in general, at least for regular color codes, however I don't think it would be all too beneficial.
Sure would be sick if players could edit their own rank colors, but what would stop people from changing their rank name to &4[Admin]? Nothing. This is very risky to do as the color of the rank is set by the group itself, not per user. I just see a lot of issues with this.

Indeed. This shouldn't be on Luckperms to do.

I know for a fact that other chat formatting plugins can do this in a regulated way. But to change at will to anything - no.

You could use LuckPerms meta combined with another plugin to do this, but sorry, out of scope for inclusion in the main plugin itself.