How to change barchart color based on frequency value

Override getItemPaint() in your XYBarRenderer, as suggested here. Use Color.getHSBColor() to generate the desired colors, as shown here and here. Starting from this example, the XYBarRenderer shown in this initial variation simply creates a static palette of blue shades having varying saturation. The updated version below dynamically scales the color saturation to reflect the dataset frequency values passed to the renderer's constructor, as shown here.

import java.awt.Color;
import java.awt.Dimension;
import java.awt.EventQueue;
import java.awt.Paint;
import java.text.SimpleDateFormat;
import java.util.Calendar;
import javax.swing.JFrame;
import org.jfree.chart.ChartFactory;
import org.jfree.chart.ChartPanel;
import org.jfree.chart.JFreeChart;
import org.jfree.chart.axis.DateAxis;
import org.jfree.chart.axis.DateTickMarkPosition;
import org.jfree.chart.axis.DateTickUnit;
import org.jfree.chart.axis.DateTickUnitType;
import org.jfree.chart.plot.PlotOrientation;
import org.jfree.chart.plot.XYPlot;
import org.jfree.chart.renderer.xy.StandardXYBarPainter;
import org.jfree.chart.renderer.xy.XYBarRenderer;
import org.jfree.data.Range;
import org.jfree.data.general.DatasetUtils;
import org.jfree.data.time.TimeSeries;
import org.jfree.data.time.TimeSeriesCollection;
import org.jfree.data.time.Year;
import org.jfree.data.xy.IntervalXYDataset;

/**
 * @see https://stackoverflow.com/a/54680803/230513
 * @see https://stackoverflow.com/a/54362133/230513
 * @see https://stackoverflow.com/a/42612723/230513
 */
public class Test {

    private static final int N = 5;

    public static void main(String[] args) {
        EventQueue.invokeLater(new Test()::display);
    }

    private void display() {
        JFrame f = new JFrame("Data");
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        IntervalXYDataset dataset = createDataset();
        JFreeChart chart = ChartFactory.createXYBarChart("Data", "Time", true,
            "Value", dataset, PlotOrientation.VERTICAL, false, false, false);
        XYPlot plot = chart.getXYPlot();
        XYBarRenderer r = new MyXYBarRenderer(dataset);
        r.setDrawBarOutline(true);
        r.setBarPainter(new StandardXYBarPainter());
        plot.setRenderer(r);
        f.add(createPanel(chart));
        f.pack();
        f.setLocationRelativeTo(null);
        f.setVisible(true);
    }

    private class MyXYBarRenderer extends XYBarRenderer {

        private final IntervalXYDataset dataset;

        public MyXYBarRenderer(IntervalXYDataset dataset) {
            this.dataset = dataset;
        }

        @Override
        public Paint getItemPaint(int row, int col) {
            double value = dataset.getYValue(row, col);
            Range r = DatasetUtils.findRangeBounds(dataset);
            double range = r.getUpperBound() - r.getLowerBound() + 1;
            float scaled = (float) (value / range);
            return Color.getHSBColor(2f / 3, 1 - scaled, 1);
        }
    }

    private ChartPanel createPanel(JFreeChart chart) {
        final XYPlot plot = chart.getXYPlot();
        final DateAxis domainAxis = (DateAxis) plot.getDomainAxis();
        domainAxis.setTickUnit(new DateTickUnit(DateTickUnitType.YEAR, 1));
        domainAxis.setTickMarkPosition(DateTickMarkPosition.MIDDLE);
        domainAxis.setDateFormatOverride(new SimpleDateFormat("yyyy"));
        return new ChartPanel(chart) {
            @Override
            public Dimension getPreferredSize() {
                return new Dimension(500, 250);
            }
        };
    }

    private IntervalXYDataset createDataset() {
        TimeSeries series = new TimeSeries("Value");
        Calendar c = Calendar.getInstance();
        for (int i = 0; i < N; i++) {
            series.add(new Year(c.getTime()), i + 1);
            c.add(Calendar.YEAR, 1);
        }
        for (int i = 1; i < N; i++) {
            series.add(new Year(c.getTime()), N - i);
            c.add(Calendar.YEAR, 1);
        }
        return new TimeSeriesCollection(series);
    }
}