Suggestion: Add ability to recolour brushes

[sbrl]

When using multiple brushes, it can be confusing to tell them apart. I propose a new command like //brushcolour or something that allows you to recolour a brush item to make it less confusing.

IIRC Minetest has the ability to recolour sprites dynamically, but I can't remember and I haven't used the feature before yet.

[sfan5]

https://github.com/minetest/minetest/blob/5.3.0/doc/lua_api.txt#L654