We have published a new beta: https://dl.dropboxusercontent.com/u/13064071/plantuml.jar
With this beta, you can have:
@startuml
skinparam sequenceParticipant {
borderColor Black
borderColor<<foo>> red
backgroundColor<<foo>> yellow
backgroundColor LightBlue
}
participant Bob <<foo>>
Bob -> Alice : hello
@enduml
Is this what you are expecting ?