Here is a great link of a wartime mortar ammunition instructional poster.
http://www.ostfront.com/militaria/prods/EBAY-00030.html You will note that the German shell is ordnance tan. This second link comes from the US Army intel manual.
http://www.lonesentry.com/manuals/german-infantry-weapons/mortar-ammunition.html It describes the particular model of shell as "maroon"
I can imagine the mortar tube itself being either tan or grey. The Germans switched to armor yellow towards what would have been the end of the North African campaign. Equipment in the early days was shipped over in its original dark grey coloring. I would imagine that with the size of a morter tube being rather small, it would have been of a very low priority in the repainting lineup as the repainiting of grey items was accomplished in theatre.
As for the radio, the same story applies, only I really doubt that time or resources would have ever been committed to "repainting" an origianlly grey radio set. If you want it to be early campaign, make it grey.
Ammo cans for the MG were an accessory as well as a disposable item. again, the timeline applies.
I know that it was common to apply mud to vehicles such as tanks and trucks when tan paint was unavailable, but as for weapons systems and electronics that require cleanliness for their operability, only a very dumb man would intentinally smear dirt on them. If it were up to me, I would go with grey for the radio, and either tan or grey for the mortar. As for the ammo can, tan with heavy chipping allowing dark grey to be exposed from the chipped areas. This composition will allow the figures to be historically plausible from early to late campaign.
David