I have seen, blue, purple, black, red

What is that all about?

Color coding for different categories of content in the document.
You can change the colors to your liking here:
Tools>Preferences>Customization>Modify Highlighting
Quite useful, I think!

Yep helps you decipher between scripts, and anchor links, and plain text within <p> tags, and <divs> and so on, just helps you be able to find what you're looking for a little easier than if it were all one color.

And I definitely concur with Per, it's a very useful feature for my tired eyes

It can also help track down some coding errors if, for instance, you forgot to close a quote somewhere, and the coloring for the code following the mistake doesn't look correct.

The colored coding has bailed me out of a mess a few times. Primarily when I have been in a hurry and forgotten aquote in my markup. I test my page, it won't validate, the validator tells me I have about a million errors, and then I go back into the editor and find one tiny, missing quote in about 10 seconds because of the colors.
(yes a single missing quote will throw the validator haywire)
Color is a wonderful thing!

Speaking of colors, how do you get the FORM tags to be a different color. I was using Dreamweaver and the form tags were orange by default. I can't find them in the list when I go into the Document display properties.