Rendering BW & Color in the same image


please I need a simple solution.
I want to combine a color and BW render based on some mask maybe?
See the image

I want to render some object (like gizmo lines and such) in color while the rest of the image BW.
However, I do not want simply overlap, so the color object goes through the rest

In the image I achieved the desired effect in GIMP, but I want to do in compositing.

Multiple tnx!

OK, I figured out. Just playing the the Z-composing

Or Converter->RGBtoBW and a File-Output-Node