I just wondered if it would be better to have the active state white or yellow. So which of both would you consider active at the moment?
Well, I think there are a few factors that need to be considered. What does the rest of your layout look like? What colors are you using? Where are these buttons placed? All of those things (and more) will come into play when your users are trying to determine whether the button is active.
My suggestion would to do some AB testing with your potential users.
I kind of feel like if you have to ask, then you should be doing more to visually indicate which buttons are active than just using color. Google has some good guidelines along these lines: http://www.google.com/design/spec/components/buttons.html#
I will say I can't tell the active state by the contrast of yellow and white. Maybe add some shadow/darker border to indicate a pressed button. If use the visual indicator as a states indicator, keep in mind the visual style should be consistent with the rest of the pages. Different visual styles will confuse users.