Here is a swing example that displays three sliders on the frame and show RGB colors.

import java.awt.*;
import javax.swing.*;
import javax.swing.event.*;
import javax.swing.plaf.basic.*;

 class Slider extends JSlider{
    private String type;
    private RangeSlider ui;

    public Slider(int min, int max, String type){
        super(min, max);
        this.type = type;
    }
    public void updateUI(){
        ui = new RangeSlider(this);
        setUI(ui);
        updateLabelUIs();
    }
    public RangeSlider getUI(){
        return ui;
    }
    public String getType(){
        return type;
    }
}
class RangeSlider extends BasicSliderUI{
    public Color[] gradient = new Color[256];
    public RangeSlider(JSlider B){
        super(B);
    }
    public void updateGradient(int othercolor, int othercolor2){
        if ("Red".equals(((Slider) slider).getType())){
            for (int i = 0; i < gradient.length; i++){
                gradient[i] = new Color(i, othercolor, othercolor2);
            }
        }
        if ("Green".equals(((Slider) slider).getType())){
            for (int i = 0; i < gradient.length; i++){
                gradient[i] = new Color(othercolor, i, othercolor2);
            }
        }
        if ("Blue".equals(((Slider) slider).getType())){
            for (int i = 0; i < gradient.length; i++){
                gradient[i] = new Color(othercolor, othercolor2, i);
            }
        }
        slider.repaint();
    }
}
public class ColorSlider extends JFrame implements ChangeListener{
    Slider redSlider = new Slider(0, 255, "Red");
    Slider greenSlider = new Slider(0, 255, "Green");
    Slider blueSlider = new Slider(0, 255, "Blue");
    JPanel colourDisplay = new JPanel();

    public ColorSlider(){
        setVisible(true);
        setSize(300, 300);
        setDefaultCloseOperation(EXIT_ON_CLOSE);
        setLayout(new GridLayout(4, 1));

        redSlider.addChangeListener(this);
        greenSlider.addChangeListener(this);
        blueSlider.addChangeListener(this);

        add(redSlider);
        add(greenSlider);
        add(blueSlider);
        add(colourDisplay);
    }
    public void stateChanged(ChangeEvent e){
        int red = redSlider.getValue();
        int green = greenSlider.getValue();
        int blue = blueSlider.getValue();

        redSlider.getUI().updateGradient(green, blue);
        greenSlider.getUI().updateGradient(red, blue);
        blueSlider.getUI().updateGradient(red, green);

        this.colourDisplay.setBackground(new Color(red, green, blue));
    }
    public static void main(String[] args){
        new ColorSlider();
    }
}

Here is a swing example that displays three sliders on the frame and show RGB colors.

import java.awt.*;
import javax.swing.*;
import javax.swing.event.*;
import javax.swing.plaf.basic.*;

 class Slider extends JSlider{
    private String type;
    private RangeSlider ui;

    public Slider(int min, int max, String type){
        super(min, max);
        this.type = type;
    }
    public void updateUI(){
        ui = new RangeSlider(this);
        setUI(ui);
        updateLabelUIs();
    }
    public RangeSlider getUI(){
        return ui;
    }
    public String getType(){
        return type;
    }
}
class RangeSlider extends BasicSliderUI{
    public Color[] gradient = new Color[256];
    public RangeSlider(JSlider B){
        super(B);
    }
    public void updateGradient(int othercolor, int othercolor2){
        if ("Red".equals(((Slider) slider).getType())){
            for (int i = 0; i < gradient.length; i++){
                gradient[i] = new Color(i, othercolor, othercolor2);
            }
        }
        if ("Green".equals(((Slider) slider).getType())){
            for (int i = 0; i < gradient.length; i++){
                gradient[i] = new Color(othercolor, i, othercolor2);
            }
        }
        if ("Blue".equals(((Slider) slider).getType())){
            for (int i = 0; i < gradient.length; i++){
                gradient[i] = new Color(othercolor, othercolor2, i);
            }
        }
        slider.repaint();
    }
}
public class ColorSlider extends JFrame implements ChangeListener{
    Slider redSlider = new Slider(0, 255, "Red");
    Slider greenSlider = new Slider(0, 255, "Green");
    Slider blueSlider = new Slider(0, 255, "Blue");
    JPanel colourDisplay = new JPanel();

    public ColorSlider(){
        setVisible(true);
        setSize(300, 300);
        setDefaultCloseOperation(EXIT_ON_CLOSE);
        setLayout(new GridLayout(4, 1));

        redSlider.addChangeListener(this);
        greenSlider.addChangeListener(this);
        blueSlider.addChangeListener(this);

        add(redSlider);
        add(greenSlider);
        add(blueSlider);
        add(colourDisplay);
    }
    public void stateChanged(ChangeEvent e){
        int red = redSlider.getValue();
        int green = greenSlider.getValue();
        int blue = blueSlider.getValue();

        redSlider.getUI().updateGradient(green, blue);
        greenSlider.getUI().updateGradient(red, blue);
        blueSlider.getUI().updateGradient(red, green);

        this.colourDisplay.setBackground(new Color(red, green, blue));
    }
    public static void main(String[] args){
        new ColorSlider();
    }
}