GreySkull EX9 Filter 1.1 has a saturate() for Custom technique.
Photographic images may require something more than simply removing the saturation, so I made GreySkull (EX9.Texture Filter), a convenient filter to convert textures to gray scale.
It's an effect, so you can best it the way you want.
It comes with more than five, though less than seventy-eight techniques; to be precise, a nice, round ten tenchniques. One of them lets the user choose how to translate the single RGB values into gray values.
I made dis, because typing gray or grey into search box did not return any result - yes, even after clicking on the very search button, for those wondering.