...That junction on the back side is missed SO many times.
I like your suggestion. A couple of years ago I came to that junction, lost in thought, and headed down (I was on my way to the summit). I'd done Whitney many times, but wasn't paying attention, and since the trail over from Trail Crest pitches down for awhile ... Anyway, it dawned on me when came to the first tentsite on the right (the one with the rock walls for windbreaks) that there weren't any tentsites along the trail.
I don't know that the sign has to be in red. If it were up to me, I'd post one about 50 yards down the trail towards Guitar Lake that says something like "This is NOT the Mt Whitney Trail. If that's your destination, go back uphill to junction and turn left" and make it visible to downhill hikers.
I'm partial to those steel signs with the lettering cut thru (laser?). They seem to last forever, and you never have to worry about the lettering wearing out. You can also give them a whack with a pole or ice axe if they're snowy/rime iced-up without damaging them.
