jtools combines several options into the colors argument in plotting functions.

Details

The argument to colors in functions like effect_plot, plot_coefs, and others is very flexible but may also cause confusion.

If you provide an argument of length 1, it is assumed that you are naming a palette. jtools provides 6 color palettes design for qualitative data. 4 of the 6 are based on Paul Tol's suggestions (see references) and are meant to both optimize your ability to quickly differentiate the colors and to be distinguishable to colorblind people. These are called "Qual1", "Qual2", "Qual3", "CUD", "CUD Bright", and "Rainbow". Each of the "Qual" schemes comes from Paul Tol. "Rainbow" is Paul Tol's compromise rainbow color scheme that is fairly differentiable for colorblind people and when rendered in grayscale. "CUD Bright" is a brightened and reordered version of Okabe and Ito's suggestions for 'Color Universal Design' while "CUD" is their exact scheme (see references). "CUD Bright" is the default for qualitative scales in jtools functions.

You may also provide any color palette supported by RColorBrewer. See all of those options at RColorBrewer::brewer.pal()'s documentation. If you provide one of RColorBrewer's sequential palettes, like "Blues", jtools automatically requests one more color than needed from brewer.pal and then drops the lightest color. My experience is that those scales tend to give one color that is too light to easily differentiate against a white background.

For gradients, you can use any of the RColorBrewer sequential palette names and get comparable results on a continuous scale. There are also some jtools-specific gradient schemes: "blue", "blue2", "green", "red", "purple", "seagreen". If you want something a little non-standard, I'd suggest taking a look at "blue2" or "seagreen".

Lastly, you may provide colors by name. This must be a vector of the same length as whatever it is the colors will correspond to. The format must be one understood by ggplot2's manual scale functions. This basically means it needs to be in hex format (e.g., "#000000") or one of the many names R understands (e.g., "red"; use colors() to see all of those options).

References

Paul Tol's site is what is used to derive 4 of the 6 jtools-specific qualitative palettes: https://personal.sron.nl/~pault/

Okabe and Ito's palette inspired "CUD Bright", though "CUD Bright" is not exactly the same. "CUD" is the same. See http://jfly.iam.u-tokyo.ac.jp/color/ for more.