First year using borage as slash mulch, does it have to be green to slash?

My plan was to keep the borage alive and well through the season just for its use as a great companion plant, then right before first frost I’d take a machete and do the hard thing to these beautiful plants, chop that up and just scatter it around.

Does it have to be green to get the full benefits, though? Is right before first frost a good time, since it’d be dying after that anyway? Does anyone have any resources for research? My quick googling didn’t bring up much for some reason.