Should a button become lighter or darker on hover?

The key is not really the colour, it is the contrast.

Let say that you have the more common scenario of dark text and no other part of the design changes on hover and it's the background only, then when you hover, the button should be given a highlight. You're trying to focus on an item - to examine it and therefore it makes sense to brighten it up as if giving it more light to see by. Appearing backlit or frontlit makes no difference.

By comparison, look at the inverse scenario, where you have light text, the same rule of thumb would say that in order to increase relative contrast between figure and ground, the background should darken, to increase the relative contrast.

Source: ux.stackexchange