Umm...
I tried making a library at one point- let's just say that it compiled and added to my program properly.
This page may be of some use, it has links to the LibraryTemplate and LibraryBasics- which is very useful.
I also looked at the controlP5 Source to see how the best library makers do it.

Firstly, you have to use all of those java modifiers- public, private, protected, static, final, and so on.
Even if you added the processing.core package to the project, you still need to call import processing.core.*; at the very beginning of your code, after package myPackageName;. Then, within your class, you need a PApplet, typically called parent. The library's constructor should receive a PApplet that you then assign your parent to. When you do all of your function calls as you would see in the PDE, you have to call those on parent.
So in the PDE, you might say
background(255);
In your library, you say instead
parent.background(255);

Also, as you mentioned above, the color datatype... is just an int. I think. Not really sure about that.

This probably seems overwhelmingly more complicated than you expected it to be, so feel free to ask more questions.

See also the Processing in Eclipse tutorial.

Thank you for your help with links and instructions. It was much appreciated, and I don't feel nearly as lost.