I watched "Rethinking Color and Contrast", a talk by Jared Smith at Inclusive Design 24 in 2017.

Some takeaways:

True colorblindness is very rare; color deficiency is much more common, especially among white men (around 8% of white men, around 5% of Asian/Black men).

The WCAG contrast guidelines refer to luminance (light dark) contrast, but there are other kinds of contrast that affect ease of visual perception.

• Cold-warm contrast
• Complementary contrast - adjacent complementary colors provide increased contrast
• Simultaneous contrast - if a color is surrounded by a color, we might perceive the inner color to be the color opposite the outer color on the color wheel when it's not.
• Contrast in saturation
• Contrast of extension (aka quantity or proportion)

The WCAG contrast guidelines don't take into account font weight, font choice (serif fonts can be harder to read on the web than sans serif fonts), or multicolor backgrounds (gradient backgrounds, text on images, etc).

The formula used in the WCAG contrast guidelines to determine contrast ratios has a bias against reds and greens - they may look subjectively plenty easy to see, but the WCAG formula might say the contrast ratio is insufficient.

True white on true black/black on white has the highest possible contrast ratio, but can be hard to read, especially for people with dyslexia.

People might feel a subjective difference in how easy to read different color combinations are, even if they have the same WCAG contrast ratio.

You can put red text on a green background and come up with a 4.5:1 ratio, and so someone who is red-green colorblind would still be able to adequately read the text.

We must implement common sense when working with colors.

There is a draft spec in WCAG 2.1 for graphics contrast, which currently is not covered by WCAG criteria.