Apparently, the issue has to deal with your DWF settings. I exported a sheet to DWF from the model and it appeared black and white again... When I exported a 2nd time I checked the print settings and for some reason, it was set to Black and White. I change the setting to color and the DWF appeared in color when I brought it into 360 Docs.
I saved my model and republished it to BIM 360 Docs and checked the sheets again. Now everything appears to be good.
Check your DWF printer setting if you have the same issue.