I was confirmed as an adult some years ago (but otherwise was a cradle Catholic).
I chose to wear a light coloured suit but only because I already had one. Nobody suggested to me what I should wear and I have never heard of any rules about attire at Confirmation - other than the normal ones regarding modest and appropriate clothing while being in a Church…
Just wear your ‘Sunday Best’ if you don’t want to go to the expense and effort of buying something specially for the occasion, it’ll be just as good.