How should I indicate that a button is clickable on mouseover?

I have white buttons on a blue background:

When moused over, I change their background colour to blue to acknowledge this:

When clicked, they become the large square you see on the left.

It's occurred to me that making the button the same colour as the background would imply that it's not clickable, and would maybe be better as a disabled style.

What's a better indicator?