Hi there,
the colors for the categories are defined in the Customizer > Additional CSS, there is a CSS rule for each of the categories, and each one looks like this:
body:not(.single) .post-term-item.term-business {
background-color: var(--business);
}
If you add a new category term to your site, then you need to create a new category rule.
eg, i have a category of cake
you would add:
body:not(.single) .post-term-item.term-cake {
background-color: var(--your-color-here);
}
Where it says --your-color-here
you would change that for a color variable name that you have set in Customizer > Colors