Marlin is able to use G10 and G11 for firmware retract and un-retract. The settings for those are set with M207. Hardware retraction using G10 will retract ALL extruders the same amount you specify.
I have been using the Diamond hotend on Reprap firmware and would suggest 3mm retract/un-retract with 0.5-1mm extra restart distance.
@agiachino If you can add the option to use G10/G11 in place of any retract/un-retract commands that would be great.
The biggest problem I’m having with all of the color mixing heads I’ve used is blobbing. The other filaments that aren’t being used are sitting in the heat and as they get more liquidy the filament in use is able to push into it and force out a blob of extra liquidy filament, which comes out all at once as a blob and ruins the print. As of yet I’ve found no real way around this issue.