I have searched a lot but not getting the idea of
using secondary color
as per 60-30-10 rule in web design...
Where to use secondary color in website???
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I'm not sure if it will be helpful or not, but Google-material-design guidelines sometimes help to catch things up even when you're not building something that follow material design methodology. I hope, at least you might find some basement.
I want to mention this special section in material design guideline, where you might get some idea of where to use primary color and where to use secondary -> Material-design-color-system. Read thoroughly from this point to the rest of the section, you may find some good idea :)
Besides, using this color tool helps sometimes -> Material-design-color-tool
IMHO, primary colors and secondary colors are important to visualize the information/content on the screen in a better way. So, it actually depends on how you organized your information/content earlier. This is a approach which makes usage of colors and typography very bold and precise. It's all about how we differently work with information, right?
Maybe that's why there's no definitive solution of where to use primary colors and where to use secondary, and where to use tertiary if it has one.