That's a good idea, many thanks for the suggestion.
So we have implemented a new function reverse_color in last beta http://beta.plantuml.net/plantuml.jar
We are using a different implementation for reversing the color (see here).
We have also put a temporary reverse_color2 function which does negate the color using code in Line 65 of ColorOrder.java.
We let you test both functions so that we tell us which one is more useful or if we have to keep both of them, probably renaming reverse_color2 to something else.
Thanks for your help !