Polymer Clay is pigment based and follows the same "rules" as all pigment based color theory and color mixing.

https://99designs.com/blog/tips/the-7-step-guide-to-understanding-color-theory/

https://www.canva.com/colors/color-wheel/

https://mymodernmet.com/basic-color-theory/

https://blog.psprint.com/printing/134/

Make a Palette:

https://coolors.co/

From a photo:

https://www.canva.com/colors/color-palette-generator/

more complex:

https://paletton.com/#uid=1000u0kllllaFw0g0qFqFg0w0aF

Use an App:

https://www.polymerclayer.com/colormixr.html