Depending on the positon in file "skinparam DefaultFontColor" overwrites the other font colors, eg the "red container" should remain red
@startuml
' here it works
' skinparam DefaultFontColor blue
skinparam rectangle<<RedContainer>> {
StereotypeFontColor red
FontColor red
BackgroundColor lightred
BorderColor darkred
RoundCorner 0
DiagonalCorner 18
}
' here it fails
skinparam DefaultFontColor blue
rectangle "==red container" <<RedContainer>> as rC
rectangle "==default container" <<container>> as r
@enduml

In older versions it was working
BR Helmut