Currently it's not supported to change grading for intersections individually. However, if the intersection material would be the same as one of the road, you can try change grading material from the whole road level. Hope it helps.
Have you tried out both two roads? I'm not sure which road's material would this intersection taken from.
Hmm, here's how to change material in the road level, for your reference:
1) right click the road -> make sure "Show Roadside Grading" is checked
2) click road centerline to show road stack
3) change material under "Grading" section at road stack
Try both roads till intersection assigned with road materials. Hope it helps.
I propose that in 3d civil be joined to the main corridor so that it is on the same surface and at the moment of import in Infraworks should be put to the same style. This happens because in Civil 3D when creating an intersection asks you if you want a new or existing one to one
Hmm, I am likely missing something, but when I right-click on my road, I don't have the "Show Roadside Grading" option you mention. I have a component road.