Been to comet factory in Italy and as far as I am aware the grey is the Acid pump only. I saw only three pump models with a grey body all are acid only and used for corrosives. "Configuration: 2 diaphragms in NBR on standard versions (Desmopan ® on request), Viton on acid versions. Parts in contact with spray liquid: AISI 316-L stainless steel, polyamide (standard versions black), polypropylene (acid version grey). Standard: regulation valve (Available in two types). Can be fitted directly to gas engine with reduction kit.(¾ or ⅝" Shafts)".