Pages Invoice Template

Arrange menu > Section Masters > Make Master Objects Selectable. Now both the top and bottom rules can be selected, and the color changed. When you are done, Arrange menu > Section Masters > Make Master Objects Selectable (to deselect this feature). Once again, these lines become uneditable, background objects throughout the current section (and subsequent sections if you have Document > Section tab > Match previous section selected) in those following sections.

You are welcome.

You might want to augment with the free, Apple authored, Pages User Guide for Mac iBook. They appear to update it as new versions of Pages arrive and it was last updated Sept 30 for Pages v8.2.