Tape off and cover the yellowed areas,then place the board in the sun for 8 hours.
Jesus, I like it! Probably won’t do it, but I like your ingenuity!
I’m hoping I got a smile out of ya. 
The only way I’ve seen this happen is with either bad mixing or ratios off. The only way it can happen this bad. Wish I could say I’ve never had a brain freeze where I mixed it wrong and was positive that I had. But chemistry is chemistry and it doesn’t lie. When certain things happen it can only be that thing. 10% off is a problem 5% not so much. Nice that it went off OK and color was the only downside. I had a guy come in one time and put two gallons in front of me … guy was PISSED because the resin never went off. I pointed out that both gallons were hardener. Oh well … Sorry for the problem … Scales are fine but the mix ratio is 45 to 100 which can be difficult sometimes and mstakes can be made. I like using ratio pumps from michaelengineering.com. Those are very accurate and are the easiest way to measure quantity. The Sticy Stuff despenser is only about $250 and does a great job.
Ice 9 has 8 foam densities and three foam formulations including 2 MDIs. My personal experience with MDI foams (in general) is that they tend to yellow more easily. Even when using the old Clark foam (TDI) and shaping outside, I had times when it was turning yellow before my eyes. I set a sanding block on the blank and had a lunch break. There was a light area where the block had been sitting.
I have two pieces of a board (Clark Foam) that was glassed a few years ago with Resin Research. It’s been outside all this time and still has yellowed less than a polyester glassed board.
To be certain, take some polyurethane foam scraps (this foam and others if you have them) and glass a couple with poly and a couple with epoxy. Leave some unglassed. Put a rubber band or something around them to approximate your straps. Leave 'em outside for a couple of days. I’ll bet you find a white line on all of them after just a few hours exposure. I think you likely have one of the MDI formulations.
I like the idea of shading the rest of the board and giving the stripes a localized suntan.