It's a food dye, if you can eat it you certainly can put it on your skin. And they probably did but don't want to announce that they experimented on themselves.
That doesn’t always follow. There are (organic, large-molecule) substances that your mucus membranes will protect you from, and which you’ll then digest (denature) and greatly metabolize (non-leaky intestinal absorption routes through the liver) and so render harmless; but which would greatly harm you if left on your skin, as the substance can potentially absorb from there all the way to your bloodstream, without any digestion or a chance at first-pass metabolism.
Think, for example: testosterone gel. Eating it wouldn’t result in much if any testosterone entering your bloodstream — but rubbing it on yourself sure does.
Rendering transparent rather than covering up would be kind of an anti-cosmetic use.
The reason I mention cosmetics, is it means that it's at least in theory skin-safe for humans up to at least some threshold. The next questions would be: what's the threshold at which it's not skin-safe for us anymore, and is that high enough for this effect to happen? Neither are foregone conclusions though.
Also basic human biology: just because something's safe to consume (which keeps it isolated from the rest of your body until it's been through the digestive tract) in extremely low doses doesn't mean it's safe to expose yourself to a highly concentrated dose via external exposure.
Did the article imply you need red light to see the effect though?