I used that effect for decades on the local site I hosted for artists here in Placitas. But of course I had to use two images, grayscale with color on the mouse over layer. This is a much better solution.

The site is coming down soon and I no longer host the site but here is one example: http://placitasartists.com/gallery_09.html

On the home page I use a similar effect. The images are in color but the saturation is cranked back. On mouse over the color goes back to full saturation. Is this possible with this CSS approach?