Pen 50 is the poché pen. It is gray (80% +/- I think) in model and layout pen sets. It should be the background color of any cut fill in new construction. (Existing condition elements are white.)
That said, one of the advantages of pen sets is having black+grayscale output while the much more colorful model pens help you stay organized. So we have an alternate poché pen, #70, which is sort of tealy in model but looks exactly like pen 50 in layouts.
Lately I've been using them for other distinctive conditions.
Complicated 'patching' profiles. The color makes clear where the profiles meet the conventional elements.