With Isadora 2 on my Mac, I was using a ffgl plugin called " ffglcolourpass ", that I found in Resolume if I remember well, wich allowed to select a specific color existing in a video, for example a red rose, keep it, and output the rest in black and white. ( in my case it was more like blood ;-) ) 
On Isadora 3, this plugin does not work. I try to put it into the directory FreeFrame_x64 but of course I guess it was hopeless if it's a 32...

Any idea of a replacement effect or a workaround?