Hi there.
Answer 1:
There are color classes like color-1, color-2, color-3... I think there's even color-8. To make everything inherit the primary color, you have to edit the Home page with SP Page Builder, and in each add-on, go to the Style tab. In the CSS Class field, delete that class. You should find, for example, color-2. Delete only that class, not all the classes in the field.
Answer 2:
I'm not sure if you thought this through. In your first question, you're asking how to remove colors and make everything look uniform, and in the second, how to make each category label in the "Featured" section have the color of the section that displays articles from that category. Now, assuming you decide to continue using colors, it's difficult to make the category in the featured articles have a different color because you need a selector, and there are two ways to do it:
1. Modify the PHP layouts to add the category name as a selector and style that selector. If you add or edit a category, you'll need to edit the CSS that colors those selectors.
2. Using JavaScript, insert a selector based on the category name. In that case, if you change or add a category name, it will no longer be colored as desired.
Regards