Color

https://www.colorhexa.com

CSS colors: https://www.w3schools.com/cssref/css_colors.asp

CSS Color Module Level 3: https://www.w3.org/TR/css-color-3

Sorted CSS colors: https://enes.in/sorted-colors/

Color palette generator with machine learning: https://news.ycombinator.com/item?id=30529262 - https://huemint.com

Pastel, a command-line tool to generate, analyze, convert and manipulate colors: https://github.com/sharkdp/pastel

Coloring for Colorblindness: https://davidmathlogic.com/colorblind/#%23D81B60-%231E88E5-%23FFC107-%23004D40

HSL

https://developer.mozilla.org/en-US/blog/learn-css-hues-colors-hsl/

Palettes

https://meta.stackexchange.com/questions/390761/colors-update-a-more-detailed-look

https://stackoverflow.design/product/base/colors/

Convert color with opacity/alpha to color without

https://filosophy.org/code/online-tool-to-lighten-color-without-alpha-channel/

Also see:

https://stackoverflow.com/questions/7741099/remove-the-alpha-from-a-colour-but-retain-its-texture

https://stackoverflow.com/questions/746899/how-to-calculate-an-rgb-colour-by-specifying-an-alpha-blending-amount