I just checked and it only lightened up if I set the purchase price to $9.99. I then did another check for info from itch (I have a hard time figuring out words) and it seems to be a commonly requested feature that hasn't yet been implemented. They suggest paying full price and asking for a refund :s
I'll just wait until next month when I can fully afford it.
https://itch.io/t/2473116/pay-the-difference-or-pay-again
https://itch.io/t/1489111/question-regarding-pay-what-you-want-price-and-upgradi...