Mine started yellow, changed to blueish-purple for months and has now started back to yellow.
Best explanation was that they sometimes shift color to match other fish. In my case it was mostly wrasses of red and blue hues. I’ve since added a CBB and long nose butterflyfish (yellow) and maybe the Midas has decided it’s cool to be yellow again.
YMMV but the color shift looks different than the stress colors that I find are blotchy.
edit to add: the coloration varies regionally too. African vs Maldives iirc