Donna, I'd say you could use semi-transparent colors for glazing. I do. The color for the shadows you are looking for is ultramarine blue deep from old holland, you can glaze with that shadow sides of the mountains, shadows in the landscape, still life shadows etc. (Check the thread 'What art materials have bought recently' I just got two more tubes of it for the summer's pleinairs.) You can mix a dot of any red to turn it to little bit reddish but I lately use also raw umber. With that mix you get a real dark mix. If there's a greenish shadow then you can maybe try just a tiny bit of ochre mixed with that blue.