Email client with Send Later and colour-coded flags features Is there an OSX email client, which allows schedule sending emails and has colour-coded flags?
This function is less about the client and more about the server supporting server side rules. Send later needs both to agree on that functionality. Color coding is certainly a client side feature and possibly server side if you expect those to sync to other clients or webmail. 

One client certainly has all of what you ask:


Microsoft Outlook - https://itunes.apple.com/us/app/microsoft-outlook/id985367838?mt=12


One such combination you'll want to check is Microsoft O365 Subscription and Outlook 2016 for Mac. It works really well for sending later and the color categorizations that mimic the colored flags.

Tons of automation and other features are available as well including server side rules and archiving, etc... but it hits your two main requirements out of the park for several organizations as well as individuals I support.

You could also set up AppleScript automation to fire and launch mail and send a message, but that would be pretty complicated to set up and maybe not as reliable as outlook.  I’d start there for free trial or for a month paid to see if that suits your needs.