Using the following diagram generates three colored edges instead of a single edge with 3 colored lines:
@startuml
skinparam colorArrowSeparationSpace 3
start
:do a;
-[#blue;#yellow;#green]->
:do b;
@enduml
If I remember it correctly it would generate something similar to this (made with graphviz):
@startuml
digraph G {
a -> b [color="red:blue:green"]
}
@enduml
I don't know if it's a bug or I'm remembering it wrong.