Nimbus uses instances of the Painter interface to paint components. With each Swing component it associates a foreground and a background Painter, and there may be several painters for different component states.

Nimbus allows customizing many of its properties, including painters, by altering the UIDefaults table. Here's an example:

    UIManager.put("ProgressBar.tileWidth", myTileWidth);
    UIManager.put("ProgressBar[Enabled].backgroundPainter", myBgPainter);
    UIManager.put("ProgressBar[Enabled].foregroundPainter", myFgPainter);

Per-component customization is also possible. When rendering a component, Nimbus checks its client property named "Nimbus.Overrides". The value of this property should be an instance of UIDefaults. Settings from that table override the UIManager settings, but for that particular component instance only. An optional client property, "Nimbus.Overrides.InheritDefaults" of type Boolean, specifies whether the overriding settings should be merged with default ones (true), or replace them (false). By default they are merged:

    JProgressBar bar = new JProgressBar();
    UIDefaults overrides = new UIDefaults();
    overrides.put("ProgressBar.cycleTime", 330);
    ...
    bar.putClientProperty("Nimbus.Overrides", overrides);
    bar.putClientProperty("Nimbus.Overrides.InheritDefaults", false);

Colors in Nimbus are derived from a core set of primary colors. There are also secondary colors, which are derived from primary ones, but serve themselves as base colors for other derived colors. The derivation mechanism allows for runtime customization, i.e. if a primary or secondary color is changed, all colors that are derived from it are automatically updated. The method NimbusLookAndFeel.getDerivedColor(java.lang.String, float, float, float, int, boolean) may be used to create a derived color.

These classes are designed to be used while the corresponding LookAndFeel class has been installed (UIManager.setLookAndFeel(new XXXLookAndFeel())). Using them while a different LookAndFeel is installed may produce unexpected results, including exceptions. Additionally, changing the LookAndFeel maintained by the UIManager without updating the corresponding ComponentUI of any JComponents may also produce unexpected results, such as the wrong colors showing up, and is generally not encouraged.

Note: Most of the Swing API is not thread safe. For details, see Concurrency in Swing, a section in The Java Tutorial.