With that plant in particular (Kichijokan and its variegated derivatives, including Kissho Kan) the red color is associated with new, active growth. So I would expect to see it strongest and brightest when a plant is doing well, like in the zone, which probably relates to your experience Stush. And yours as well Steve, that plant looks really happy.
I am a sucker for bright colored spines (these reddish ones start out yellowish orange in bud) and am always on the lookout for picture-taking. This little detail is actually quite informative about the status of several different agaves. So it is worth paying attention to.
I put both Kichijokan (all blue) and Kissho Kan (marginal variegate) in the public garden, where they are now getting by on rain alone. The spines at the top are a very different color at the end of the summer drought compared to winter, when they get rain. And the variegate is wayyyy slower and smaller growing than the wild type.
