You select the area of the render with pen tool, lasso tool or whatever, and once you did that, hit control + shift + I, on windows, that inverses the selection, selecting everything but the area of the render that you've selected earlier.
Now just desaturate it, control + alt + shift + B, that brings up the Black and white box, okay it, there you go. The background is desaturated, but the render remains as it is.
Or you can try this way, though you might need to be precise.
First, Desaturate the image, entirely, then you take your pen tool and make a selection around the render, and once you do it, Take the History brush tool and paint over the selected part simply.
Two ways.