George,
I can offer you only my knowledge of having seen one of the alledged originals.
In Montjuic Fortress, above Barcelona, around 30 years ago they had an 88mm suppossedly from the Condor Legion.
When I saw it, it was in a faded grey (and rust having been outdoors for a long time) and did not have a gun shield.
The colour was not the usual 'Panzer Grey' or the Reichswehr three tone camo, but a Luftwaffe colour called Anthrazite Grey. Vehicles were also supplied in this colour too;
While most ordnance sent to Spain with Condor was in the three-tone, the 88s seem to have come from Luftwaffe stocks, and the eventual five batteries seem to have been laregly in the grey, though it is possible that the original 'Aldinger' battery may have had three-tone camo'ed guns when sent in 1936.
This seems to be confirmed by a photo of them when on display before leaving Germany;
...and that book is well worth getting. Over 130 photos.
So in summary, you can do them in Reichswehr three-tone or Anthrazite Grey.
Hope that helps.