Alright, here's some questions I have in my head while I'm reading this...
1. If it was a "core shot", then wouldn't you see the bone/ yellowish color in the core/middle of the bait? Bass-boys posted a picture of a cut up Creme worm (thanks for the picture by the way @Bass-Boys) and the core is not a different color.
2. If it was "dyed" after the fact, wouldn't the dye spread throughout the worm, eventually turning it all red? Say it's a non bleed dye, wouldn't the core be that bone/yellowish color? How do you explain the consistent fade? I've only seen airbrushing do that kind of fade and I doubt that's how they did it.
I like what @Basseducer said but the measurements and how much you inject would have to be PRECISE. It's not impossible though. I might have to give it a try and experiment.