Hydrangeas are kind of cool, colour-wise. You can get a white variety that stays white until the bloom starts to age and then it will turn green. The common variety is one that will be either pink or blue depending on the PH of the soil. I don't remember exactly what it is that turns pink ones blue, but I think it's aluminum in the soil. Also, the coloured ones (pink or blue) will eventually change to green as they age, although they might go through a mixed colour stage with white, pink (or blue), and green petals. Apparently, how green they get depends a bit on the weather and how hot it is. Interesting, eh?
Thus endeth the lesson for today!