I don’t think my solution will work though, but it’s worth a shot.
I would do all of that manually though. It shouldn’t be too difficult to change all of the CSS colours if there aren’t too many. Let’s say they have 10 different colours; You can search/replace the values for different values. I don’t think that should take too long. And also, I’m not sure how many images there are, but what you could do is drag them all into photoshop and create an action that desaturates and then closes and saves the image. So all you do is click "play action" over and over again. The action would be completed practically immediately and you would just be clicking away completing more than 1 a second. After that you just upload the images again. (There is an EVEN quicker way of doing this in photoshop but I’ve forgotten how)
p.s. That sohtanaka.com example you showed doesn’t actually make the image gray. It just fades a colour image into a gray image, so there are 2 images.