(I must of used a temp login and it's not letting me edit the above with a registered login with same id)
OK - you can customise individual elements via the it's <<.stereotype>> but only certain objects can have this, and I don't believe groups are one of them.
Luckily there is the following syntax to work around this limitation::
group#Blue #LightBlue Blue
group#Red #violet Red
Found in https://forum.plantuml.net/6410/can-we-format-groups-or-have-different-styles-for-group-alt