We know from the pictures of the broom relay that Luna Nova didn’t have different-colored robes during Croix and Chariot’s time as students as opposed to now. They were always dark colors.
The only time we saw Croix and Chariot in white robes, however, was during the flashbacks of episode 22, when they appeal to Woodward for the Grand Triskelion. They have already been blessed with the robes, probably by Professor Woodward herself, as it seems only a professor or a much more experienced witch can provide those special robes for you.
Croix and Chariot were blessed with those white robes at what was supposed to be the moment of their success, just like Akko and company were later blessed by Chariot and Croix.
Which means that this was the exact second Chariot and Croix COULD HAVE unlocked the seal, but due to Croix’s jealousy/bitterness of not being given the Rod, the negative emotions far outweighed the purity required to utilize the magics, hence its rejection of her and their eventual failure to succeed.
Just the color of their robes now tells us this was exactly when they were supposed to have succeeded, but couldn’t. And ouch it hurts.