I've been designing buttons for a while now but when it comes to selecting the color(s) for the hover state of either a gradient or solid its been mostly been a matter eyeballing a darker tone. Is there a structured color theory out there that would help better judge what would be a more appropriate tone to use? How dark should I go from the original?
This may not answer the question..
When looking for a slight change in lightness I usually do the math - simple counting actually. If I have
In the case of using 0's and F's though, you may have to compromise as you can't subtract 1 from 0 and you can't add 1 to F.
Also, I don't know how this could be applied to the (255, 255, 255) format if that's what you're using.
Or, you could let some online generator do the colour choices for you, and just nick the code you need: CSS gradient Button is just one example.