Fascinating
Note that careful inspection of the images shows that the frost is not actually IN the gullies, it's on the bare rock faces between the gullies. These have less dust, less insulation. I don't know right now what this means, or what thickness of frost is required to give this sort of signal. Looks like more research is needed by Yours Truly...
I'll be interested to see how this situation evolves through time, and how it looks in different gullies at different lattitudes, especially the ones in Sysiphi Cavi which I infer to be active every martian springtime.