Just use one for both. Now to reveal why I like the method in step 2 and why it was better then just to use only black on the Outline layer. Now that you have everything color, you can now be a better judge of what color to use. Choose a color you want to use. Now just paint something.
Most people use a darker version of the color for the outline.
For example, if the hair color is brown, then use dark brown. I considered this a minor advantage but for some it can come in pretty handly. This really demenstrate the usefulness of layers. In this menu, you have the option to edit the coloring. For this example I added more blue since it's suppose to be a night scene.