Personally, color tagging is so subjective that deep down I believe we'll be forever running in circles if we hope to have it solved for good, this discussion is way longer than usual, for example.
gray_hair shouldnt deserve a special tag for shininess since the others make do with shiny_hair
gold is blonde with shiny, bronze is brown with shiny, cobalt is blue with shiny etc etc
but of these only silver has avid supporters here
While I agree with that statement and I believe many people want to see this problem settled for once, I'm not sure if deprecating the silver hair tag will make our situation better. From my own posts at least, the best examples I can think of which the tag is somehow irreplaceable are: post #5191296, post #5054278 and post #5188740. Maybe grey hair could do the job, but then it'll turn into an catch-all tag, along with everything it has been tagged with already.
I also don't think creating dark/light will be that good, it'll just pour more subjectivity into the color tagging process, though we already have 4 light_*hair* tags and 2 dark_*hair* tags and maintaining the status quo (not nuking them neither expanding them to more colors) looks like a consistency problem that make the color tagging process less intelligible and comprehensive imo.
Every shading/lighting on the hair could change a tagger's opinion, including me, one can always argue X color is just Y color with shading or shiny version of Y color, maybe that's why we have tags like platinum blonde hair. I'd prefer not deprecating the tag or creating a specific tag capable of covering that shading/lighting scenarios, unfortunately I'm not confident that using shiny hair instead would work (much less users would follow that), like we use shiny clothes, but that's the best I could think for now.