periods are:

Proterozoic: (none)

Paleozoic: Cambrian, Ordovician, Silurian, Devonian, Mississippian, Pennsylvanian, and Permian

Mesozoic: Triassic, Jurassic, and Cretaceous.

Cenozoic: Paleogene and Neogene.

Only the Paleogene and Neogene periods are divided into epochs. They are:

Paleogene: Paleocene, Eocene, and Oligocene

Neogene: Miocene, Pliocene. Pleistocene, and Holocene

Using a document-level style sheet and nested ordered lists, show this information in outline form. The eras must be numbered using upper Roman numerals, the periods must be numbered with upper alphabetics, and epochs must be numbered using decimal numbers.

The background color of the Proterozoic era must be pink. The background color of the Paleozoic era, including its periods, must be blue. The Mesozoic era must have a green background, and the Cenozoic era must have a red background.

Just to make this clear, here is a little bit of the outline:

 II. Paleozoic A. Cambrian B. Ordovician 

I did this with

 elements. You must do it with a document-level style sheet, and you must include the background colors. It is not necessary to match the color I used exactly.
 
The easiest way to do this is to make liberal use of class specifications in your document-level style sheet. So, you might code something like this:
 
 ol.eras {list-style-type: upper-roman;} 
 
If you cannot get all this to work using only a document-level style sheet, you may do the numbering with document-level styles and the colors with in-line styles, but try it with all document-level styles first, OK? It can be done.
 
If you can't get color behind the numbers, don't worry too much about it as long as there's color behind the text.
 
Name your document l3p1.html and create a link to it from your index page.