How to remove highlighting from emails in Mail?
Looks like you can do it by adding a new Rule [I haven't tested it, because I don't want to actually reset my own colours…]

Try setting Any condition > Every Message > Set Colour…



Note: the 'Other colour seems to depend on your current Colour choice in the Colours floater - Format > Show Colours…



You can do it manually, select one message, pick a colour - but it seems very hit &amp; miss, sometimes works, sometimes doesn't.