When I created a new page, I used "Heading 1" and "Heading 3" in black. When I view my page, the fonts change and the header becomes brown….????
I just posted a lengthy response to your other related question. That answer might provide some insight as to why the fonts change when you specify a different header.
tl;dr: The font size, typography, and color are determined by your theme’s stylesheet (style.css). You can view the styles by visiting: Appearance > Editor.
But be aware that applying headers (h1, h2, h3, etc.) doesn’t just change the size of your fonts. It also has SEO implications. Those headings are indexed by search engines, and priority given to the higher numbers. An H1 tag tells Google, “This is what this page is about.”
This is why it’s best to override font sizes and colors in your theme’s stylesheet using CSS.
Learn more about how to customize your theme in this video:
Hope this helps!