Thanks to a nudge from Floris, I have spent the last few days diving into RYB (Red Yellow Blue) color theory.

Wikipedia article: https://en.wikipedia.org/wiki/RYB_color_model

RYB Color Compositing: http://nishitalab.org/user/UEI/publication/Sugita_IWAIT2015.pdf

RYB is the color theory many of us were first taught in kindergarten. It's an ancient theory, developed by painters, to explain why mixing yellow and blue paint produces green. But in modern times it has fallen out of favor. It doesn't play well with the additive RGB colors of computer screens, has been replaced by CMYK for printing, and doesn't really predict how paint mixes in the real world.

Still, as the authors of that compositing paper point out, it does better for subtractive mixing than RGB does and has the virtue of simplicity. At the very least, it's handy for producing those subtractive color Venn diagrams you see in all the color theory articles.

I have added a RYB option to both my blend_color and palette nodes:

http://support.nodebox.net/discussions/show-your-work/680-new-blend...

http://support.nodebox.net/discussions/show-your-work/656-palette-node

Scroll to the bottom of these threads and download the demos to get the updated nodes. These nodes will be included in the next release of my library.

In order to test these two nodes and learn more about working with RYB, I set out to reproduce some of the figures in that compositing paper. The result is attached (see screenshot).

This demo draws six different figures; two with black backgrounds use standard RGB, the other four use my new RYB options:

• Top Left. Additive Venn diagram using RGB.
• Top Right. Subtractive Venn using RYB. Made using blend_color with Subtract Paint mode.
• Mid Left. An Itten color wheel. The secondary purple, green and orange are made using RYB blends, but the color wheel is drawn using average blending (which is closer to most modern Itten wheels), https://en.wikipedia.org/wiki/Johannes_Itten
• Mid Right. Overlapping translucent brush strokes using RYB blend.
• Bottom Left. RGB color wheel with the six fundamental RGB colors. Made using palette in RGB mode.
• Bottom Right. RYB color wheel with the six fundament RYB colors. Made using palette in RYB mode.

At first I struggled to recreate the Itten wheel using only my RYB nodes. But then I realized that this figure was included in the paper only to represent "intuitive" ideas about color mixing. The colors in the Itten wheel are somewhat arbitrary; modern versions found on the web differ from Itten's original design and vary slightly among themselves. I found that I could come close by using RYB to form the secondary colors from the primary colors, and then using the Average setting to form the remaining tertiary colors in the ring.

I also struggled to recreate the spectrum Hue vs. Saturation plots in the paper. My spectrums were off until I realized that it's not enough to set my palette node to RYB; you also have to provide the correct RYB primary colors (which differ slightly from the standard RGB ones). My two color wheels show the difference, with the primaries shown as small ovals inside the wheels.

The attached demo should provide a good start for anyone wanting to better understand my implementation of the RYB color space. Please play and share your own experiments!