The theory of colors according to Johannes Itten.
I have found a very interesting video on "color theory". One sees in this video very well, how one can influence the depth of space in a picture or a photo by means of color. It is also very interesting to illustrate how the emotional perception in a picture / photo can be controlled. Many thanks to "Der Kunsterklärer".
https://youtu.be/EdUGaS0UIhQ