A dark button means you cannot afford the tower. If you have enough gold, the button lightens up; if not, it darkens again. Additionally, if you have enough gold and select the tower, the button will turn green, indicating that it is currently selected.
Viewing post in Crystal Defender jam comments
I guess that's what I mean, to me personally it felt like the weapon was unavailable.
It could be a matter of taste, I think even changing the shade of green could help.
If I'm not looking directly at the selected weapon but rather at the screen it's really hard to differentiate the selected weapon from the other unavailable ones.
I think using a lighter color and doing something like highlighting the selected weapon with a perimeter like the orcs have around them is a good idea.