Specification for method 
decodeColor(Color color1, Color color2, float midPoint)
says
     * @param midPoint The offset between color 1 and color 2, 
     *                 a value of 0.0 is color 1 and 1.0 is color 2;

In fact midPoint value of 1.0 doesn't always lead to returning second color.
Please see the following code sample:

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import javax.swing.*;
import java.awt.*;

public class Test {
    public static void main(String[] args) {
        class MyARP extends javax.swing.plaf.nimbus.AbstractRegionPainter {
            protected PaintContext getPaintContext() { return null; }
            protected void doPaint(Graphics2D g, JComponent c,
                                   int width, int height,
                                   Object[] extendedCacheKeys) { }
            void callDecodeColor() {
                Color color = decodeColor(Color.BLUE, Color.BLACK, 1.f);
                if ( !Color.BLACK.equals(color)) {
                    throw new RuntimeException("Expected " + Color.BLACK + ", returned " + color);
                }
            }
        }
        new MyARP().callDecodeColor();
    }
}
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The output will be 

Exception in thread "main" java.lang.RuntimeException: Expected java.awt.Color[r=0,g=0,b=0], returned java.awt.Color[r=0,g=0,b=1]
	at Test$1MyARP.callDecodeColor(Test.java:15)
	at Test.main(Test.java:19)