Hello stef1 and thanks for your question. In general, there's no hard and fast rule for shadow colors. The most important thing is to get the value right, that is, the the degree of darkness of the shadow relative to the light of the bridge (or whatever might be creating the shadow.) If you catch the value accurately, you can use almost any colors you want in the shadow areas. The Impressionists played a lot with colors in shadows. When you study them you'll see that they often combined greens, blues, oranges and even pinks in some shadows. Sounds counter-intuitive, but it all works if you remember that shadows are full of light, just light of a lesser value than the bright parts. So in general, any colors will work, so long as you get the values right.