I was looking very carefully at the shadows on the wall of the building across the street. I noticed that the colours in the shadows had their own specific nature.
As a painter, I know that shadows can be obtained by mixing the local colour of the object with the appropriate complementary colour. This tends to grey the local colour tone and make it darker so you have the shadow effect. The complementary colour is usually a cool tone so you can also capture the warm-cool contrast between the colour in light and the shadow.
But I started seeing that the shadow had its own colour that didn’t depend totally on the local colour nor its complementary. In some places, the shadow on the wall seemed violet and in some places it seemed blue.