How big a sample are you testing - get a 4 x8 cheap plywood (out of the Home Depot reject pile) or several. You are on the right track with the grayed greens I think.

Has your house ever been painted with modern paints - even if it has, because its vintage and probably using vintage materials you might want to consult with limeworks.us or ,either and consider using a mineral potassium silicate paint which forms a chemical bond with the cement vs a plastic film which sits on the surface. It is much more durable, breathable - rain is kept out,, but water vapor can pass in and out which you need so moisture doesnt get trapped behind the paint film and cause damage. Because these products use real mineral pigments (vs. chemical colorants) there is a richness and depth to them that cant be matched with plastic paints. No I dont work for them! I just had a good experience with them re: what to do with my own vintage 100 yr old masonry. Beautiful house btw!