Answer 1

//This will help you, material design color tutorial
https://material.io/design/introduction/#

Answer 2

https://www.smashingmagazine.com/2016/04/web-developer-guide-color/

Answer 3

I firmly agree that every designer be it website, print or digital should atleast understand the basics of the color wheel so as to achieve harmony in their designs this is what we call Color Theory.
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For a basic detailed guide check out Better Homes & Gardens' color wheel. Making a color scheme yourself is a nice way to learn Color Theory

https://www.bhg.com/decorating/color/basics/color-wheel-color-chart/