I can't remember if I've stopped at this particular spot before, but I'm pretty sure this is a feature of the Galice Slate that's new to me- though a phenomenon that I've seen many, many times. Note the bright, highly variable, metallic colors on the cleavage surface of this stone (They are more apparent in the full-size photo). There is a spot along 199 where one can see black smoker sulfides intermingled with altered pillow basalts and breccias (I didn't spot it on this trip, which troubled and puzzled me; it's an easy spot to find, with some good landmarks), but this location is well up-section of that one, and I doubt that's what we're seeing here.
The key clue here is the presence of small reddish-brown halos and spots- there are two larger ones in the lower left middle of the face above, plus maybe a half dozen smaller ones than can be picked out at the scale displayed, and many more in the full-sized photo. My suspicion is that these are secondary pyrite crystals, likely with other metallic solid substitutions for the Fe+2, formed in a reducing sedimentary environment. After lithification, metamorphosis, exposure to surface conditions, and weathering, this pyrite oxidized, and the metal oxides and hydroxides and were distributed across available joint surfaces, including those along foliation.
I can't be certain of this hypothesis, of course, but as I say, I've seen similar "rainbow coloring" in many cases where the fresh rock contains a small to fair amount of pyrite, but which has lost its pyrite in weathered portions. The sulfur is lost as SO2, but the metallic ions have limited mobilities, and tend to precipitate on surfaces that are somewhat permeable to water, fairly close to the iron-stained spots that indicate where the original pyrite crystals have been lost.