You should learn some basic css… 
For header colors: save a file named “header-colors.css” (you can chose whatever name you like) in the .obsidian/snippets folder.
Put the following content in it:
h1 {
color: #c14343;
}
or whatever color you like (for the color codes, have a look here for example.
Repeat the same with colors h2, h3, …
This will change the color of all your h1 headings. If you only want it to change in preview mode, chose this code:
.markdown-preview-view h1 {
color: #c14343;
}