This thread post below from @Steve_Reeder might help you with rules & email account colours.
https://forum.emclient.com/t/email-account-colour-coding/35900/8
Extract thread quote from Steve Reeder.
Create a rule:
1). After message has been received
2). received using your email address
3). set categories newcategory
You can either show “category” in the column display of mail screens or under the “appearance → lists” settings, tick or untick “Use color of category as text color”
There you go… Simply to implement with the existing application. I have it set up for different colours already and it works a charm!!!