Have a look at the skinparam and play around with these:
SequenceDividerBackgroundColor
SequenceDividerBorderColor
SequenceDividerBorderThickness
SequenceDividerFontColor
SequenceDividerFontName
SequenceDividerFontSize
SequenceDividerFontStyle
so setting e.g.:
@startuml
skinparam SequenceDividerBackgroundColor red
== start ==
A -> B: hello
B -> A: bye
== end ==
@enduml