The myth of blue shadows

Recently, I have been reading through Handprint.com (MacEvoy) section on colour theory, I also found a good article by artist and blogger Paul Foxton who does amazing photo-realistic oil paintings. The consensus is that most shadows retain the same hue and same saturation as the main surface, but they have a much lower value (darker). I always added blue in shadows, but never added black because Monet never used black! To test the idea that shadows don't need blue, I painted out some examples as shown here. Ignore the crazy thing in the sky, look on the bottom two houses. For the first house, I used burnt sienna, plus indothrene blue for the shadow, while the second house on the right I used burnt sienna plus lamp black for the shadow. To my eye, the blue-containing shadow has a washed out look, as if it belongs to a overcast day, while the black-containing shadow looks bright and vibrant. The other flaw in the blue shadow theory is that yellow surfaces do not have green shadows!

The scientist in me wanted more evidence. For 30 years I faithfully added blue to shadows, avoiding black because Monet avoided black (Did you know Monet dropped out of art school?). I took a picture outside my apartment and performed a colour analysis. There must be blue hiding somewhere in the shadows. To my surprise, the hue of the shadow on a brick wall was not blue, or even purple, it was red! In fact, the hue of the shadow on a brick wall was similar to the bright red of a nearby car. In the diagram below, the shadow of the white window frame is indeed blue- its possible! But the shadow on the brick wall, seen on top, is a de-saturated, and very dark red. On the top right we see that the brick wall colour is a a partly de-saturated, grey-orange, while the car is a maximum saturation, high value red. I checked all the pixels and their wasn't a single blue one in the shadow on the brick wall. The fact is, blue light will only be visible in shadows on surfaces that are blue, white, or very near white like window frames, sun bleached sidewalks, white cars, pale white bricks, and the like. Any other surface with saturated colour, like an orange brick wall, will not reflect blue light.