Plantuml can output the names of all skin parameters by means of the `-language` option, so:
java -Djava.awt.headless=true -jar plantuml.jar -language
this give a.o. the list of skinparameters, A bit of a search gives here the following possibilities for arrow colors:
I'm not sure which one you will need and whether the right one is present (but we cannot test it either as, till now, you didn't provide an example).