Occurs with: Railgun.
Color: Pale Green.
I can confirm this. The yes button is not working.
I wasn't able to buy the green ammunition a while back, but after a day or two I was able to purchase it with no problem.
Same thing happened with micro upgrades for hornet like 2-3 updates ag
The game has been updated, please check if the issue is still there.
Customer support service by UserEcho