Hello K.,
For that you can use style, but it concerns the two diamonds (the first of the test, and the last of the 'endswitch'), you can not change colour separately, but sequentially, as:
@startuml
<style>
diamond {
BackgroundColor White/Yellow
}
</style>
start
switch (test?)
case ( condition A )
:Text 1;
case ( condition B )
:Text 2;
<style>
diamond {
BackgroundColor palegreen
}
</style>
endswitch
switch (test2?)
case ( condition A2 )
:Text 12;
case ( condition B2 )
:Text 22;
<style>
diamond {
BackgroundColor red
}
</style>
endswitch
stop
@enduml

Enjoy,
Regards.