1) Using Skinparam
@startuml
skinparam ParticipantFontColor<<bob_style>> Red
skinparam ParticipantFontColor<<alice_style>> Blue
Participant Bob <<bob_style>>
Participant Alice <<alice_style>>
Bob -> Alice : hello
@enduml
2) Using STYLE (the replacement for Skinparam)
@startuml
<STYLE>
.bob_style {
FontColor Red
}
.alice_style {
FontColor Blue
}
</STYLE>
Participant Bob <<bob_style>>
Participant Alice <<alice_style>>
Bob -> Alice : hello
@enduml