Basically I wanted to write something to highlight certain potions in my inventory (so that I didn't think I had them when I didn't).
Unfortunately, I put in the arguments backwards.
Somehow, I also had a trigger on the word "a" that coloured text in "potion" colour! (wtf?)
Anyway, the result appears to be that the letter a makes my text go black.
Deleting the highlight solve the problem.