Hey Gnu Bee,
I think your assumptions are correct. Red does seem to be a predominant color to use for tags. I know I tag my yellow sallies with a red butt. A sometimes popular steelhead fly is the red butt skunk. I also have had some of my best luck with westslope cutthroat using red humpies. I think red connotes blood to a fish, there by triggering an attack against what is perceived to be an injured or dying prey.