I'm going to take the letters one by one, one post at a time.
C is for cookie. That's good enough for me.
Except that this isn't Sesame Street (unfortunately.) C is actually for contrast. It means that the different parts of your document should look different. Headings shouldn't be the same as sub-headings, and sub-headings shouldn't be the same as body text. Sounds pretty simple, right?
I'd like to think that most people know to make headings and body text look different. I could be wrong about that, but hey, I'm an optimist. The whole point of headings and sub-headings is to help readers navigate the text. They're like road signs. "If you're headed for Chicago, take the next exit!" "If you're looking for what supplies you need for this project, read this paragraph!" Could you imagine if the road signs looked exactly the same as the road? No one would ever notice them! The GPS would tell you to turn left on Willow Road, but you wouldn't know which road was Willow Road, and pretty soon the GPS is doing nothing but scolding you with the word "Recalculating," which tends to sound very condescending in an automated British accent.
That's why road signs need to stand out, and headings and sub-headings, as the road signs of documents, need to stand out. So you make them contrast with the rest of the text.
The trouble, I think, comes from distinguishing the title from the headings and the headings from the sub-headings. These are different levels of road signs. Some things need to stick out more than others. "No parking 4 am to 6 am" doesn't need to stand out quite as much as "STOP." Likewise, the title should probably be the most prominent text on the page, followed by headings, then sub-headings, and then the main text. Really, it's very simple.