The Person Tag Add workflow action is very powerful as it not only lets you SET an existing tag on a person, but CREATE new Org Tags on-the-fly, programatically. However, such tags are always created with a default gray background color, and if you want them to have a specific color you must do so "after the fact", by manually adjusting it in the Tags UI.
It would be fantastic if the Person Tag Add action were updated to allow specifying the Background Color, just like the UI for manually-adding a tag does.
The Background Color option should probably use the default/current color if nothing is set, or override it if it does have a valid color entered. I'm not sure of it should DISallow setting of color IF the tag already exists as I don't think the (current) action even looks at existing tags... it just tries to do a match, or make a new one if no match is found.