With version plantuml-1.2022.6.jar all components which are defined without color definition (with default colors) are exported only monochrome and not the default colors.
With version plantuml-1.2021.16.jar the default colors are still printed.
Either, the setting "skinparam monochrome false/true" has no effect.
Tested with OpenJDK 15.0.2, 16.0.2 and 18.0.1