Color c = UIManager.getColor("Label.background");
There's very poor documentation regarding that. And a L&F isn't specifically required to define those colors.
Best bet is to write a small app using the L&F set to whatever you're going to use, then dump the contents of UIManager.getDefaults() to System.out and see what colors and fonts and borders exist.
Generally, you can usually find the base component type names, sans 'J': Label for JLabel, Button for JButton, followed by . and fairly common types like background, foreground, border, font