Instead of using RIT dye, I decided to start with a thin wash of black acrylic paint (Vallejo Model Air Black) followed by dry brush successive coats of lighter colors (Dark Sea Grey and Deck Tan). Following another online hint, I used some yellow ochre pigment to try to warm up the deck as a final step:
I have a lot of decking to paint, so I'll probably continue experimenting with blending the PCB and plastic together in a believable fashion.
After last night's experiment, I used the remaining gray washes to create a high tide mark on the piling wall:
