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:
ArrowColor
ArrowFontColor
ArrowLollipopColor
ColorArrowSeparationSpace
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).