Why do my color stripes get wider as I work up the body?

It's almost always a row gauge drift, not an optical thing. Row height is way more sensitive to tension changes than stitch width, so the same number of rows getting taller is the first thing that shifts when your hands relax. The fabric can still feel consistent when you pinch it because the change creeps in slowly, like a row or two of extra height per band, and you don't notice until the coral looks twice as fat.

I ran into something like this on a temprature blanket and thought I was losing my mind tracking it. What fixed it was measuring the actual fabric instead of trusting my eyes. Lay the piece flat without stretching it, measure from the hem up to the switch point of one color band near the bottom, then measure the same height in an upper band. If the numbers differ you have your answer, and you also find out exactly how much you have drifted per band, which helps you recalibrate before the yoke. Also, my drifting always got worse about an hour into a session, so I started standing up and repositioning every couple of shows and it genuinely leveled out.

I get the same thing when I've been going for hours, my hands sort of get lazy and everything stretches out without me feeling it. Tried sitting at the kitchen table for one evening instead of slumping on the couch and the bands suddenly matched again, so it might be your seating too. Could you try finishing the next color in a different chair and see if it evens up?