Colour roadmap bars based on simple formula or tag conditions

I would like to create a roadmap where bars are coloured based on a conditional check against tags in a custom field.

I use SAFe "Program Increments" as a editable custom tag field and would like to highlight my roadmap based on a condition of tags. As the Tag field can, and needs to, have multiple tags the current simple method of customizing the roadmap requires each combination of tags to be modified (see attached image)

What I'd like to be able to do is set colour by formula. For example:

IF("Program Increments") CONTAINS "PI 8" SET COLOUR BLUE
ELSE IF("Program Increments") IN ("PI 6", "PI 7") SET COLOUR GREEN
ELSE IF("Program Increments") IS EMPTY SET COLOUR GREY
ELSE COLOUR ORANGE