In fact it's working as intended. It's supposed to indicate that the upgrade is no longer available for purchase. However if I were to have had more time I'd have used a different mechanic for indicating this. We tried to get as much visual feedback into the game as possible, rather than using text.
Viewing post in GEOS: Golden Edge of the Sun jam comments
Well if it stopped at the golden background ones that would make total sense, but pressing the one that looks the most badass with the gold background will take it to a next level which looks exactly like the first level (the icon returns to the first basic one and the background turns grey) I was surprised at the first place that there was an upgrade after the one that kinda looked like it's indicating MAX LEVEL. Visual feedback is always better than text and you had some really great visual feedback in the gameplay but the shop, it looked weird.
The shop was supposed to written by Clyffe, but he was evacuated by the hurricanes and wasn't able to complete most of the code. The idea was that there would be a greyed out icon to indicate you hadn't boughten the upgrade yet, and the gold icon be the final one. As it was we ended up an icon short.