SVGLoader loader = new SVGLoader();
URL svgUrl = MyClass.class.getResource("mySvgFile.svg");
SVGDocument svgDocument = loader.load(svgUrl);

FloatSize size = svgDocument.size();
BufferedImage image = new BufferedImage((int) size.width,(int) size.height);
Graphics2D g = image.createGraphics();
svgDocument.render(null,g);
g.dispose();

class MyComponent extends JComponent {
    @Override
    protected void paintComponent(Graphics g) {
        super.paintComponent(g);
        svgDocument.render(this, (Graphics2D) g, new ViewBox(0, 0, getWidth(), getHeight()));
    }
}

g.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);
g.setRenderingHint(RenderingHints.KEY_STROKE_CONTROL, RenderingHints.VALUE_STROKE_PURE);

// Will use the value of RenderingHints.KEY_ANTIALIASING by default
g.setRenderingHint(SVGRenderingHints.KEY_IMAGE_ANTIALIASING, SVGRenderingHints.VALUE_IMAGE_ANTIALIASING_ON);

g.setRenderingHint(SVGRenderingHints.KEY_SOFT_CLIPPING, SVGRenderingHints.VALUE_SOFT_CLIPPING_ON);

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

import com.github.weisj.jsvg.*;
import com.github.weisj.jsvg.attributes.*;
import com.github.weisj.jsvg.parser.*;
import org.jetbrains.annotations.NotNull;

import java.util.Objects;

public class RenderExample {

    public static void main(String[] args) {
        SwingUtilities.invokeLater(() -> {
            SVGLoader loader = new SVGLoader();

            URL svgUrl = RenderExample.class.getResource("path/to/image.svg");
            SVGDocument document = loader.load(Objects.requireNonNull(svgUrl, "SVG file not found"));

            JFrame frame = new JFrame();
            frame.setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
            frame.setPreferredSize(new Dimension(400, 400));
            frame.setContentPane(new SVGPanel(Objects.requireNonNull(document)));
            frame.pack();
            frame.setLocationRelativeTo(null);
            frame.setVisible(true);
        });
    }

    static class SVGPanel extends JPanel {
        private @NotNull final SVGDocument document;

        SVGPanel(@NotNull SVGDocument document) {
            this.document = document;
        }

        @Override
        protected void paintComponent(Graphics g) {
            super.paintComponent(g);
            ((Graphics2D) g).setRenderingHint(
                RenderingHints.KEY_ANTIALIASING,
                RenderingHints.VALUE_ANTIALIAS_ON);
            ((Graphics2D) g).setRenderingHint(
                RenderingHints.KEY_STROKE_CONTROL,
                RenderingHints.VALUE_STROKE_PURE);

            document.render(this, (Graphics2D) g, new ViewBox(0, 0, getWidth(), getHeight()));
        }
    }
}

CustomColorsProcessor processor = new CustomColorsProcessor(List.of("myRect"));
document = loader.load(svgUrl, new DefaultParserProvider() {
    @Override
    public DomProcessor createPreProcessor() {
        return processor;
    }
});

class CustomColorsProcessor implements DomProcessor {

    private final Map<String, DynamicAWTSvgPaint> customColors = new HashMap<>();

    public CustomColorsProcessor(@NotNull List<String> elementIds) {
        for (String elementId : elementIds) {
            customColors.put(elementId, new DynamicAWTSvgPaint(Color.BLACK));
        }
    }

    @Nullable DynamicAWTSvgPaint customColorForId(@NotNull String id) {
        return customColors.get(id);
    }

    @Override
    public void process(@NotNull ParsedElement root) {
        processImpl(root);
        root.children().forEach(this::process);
    }

    private void processImpl(ParsedElement element) {
        // Obtain the id of the element
        // Note: There that Element also has a node() method to obtain the SVGNode. However during the pre-processing
        // phase the SVGNode is not yet fully parsed and doesn't contain any non-defaulted information.
        String nodeId = element.id();

        // Check if this element is one of the elements we want to change the color of
        if (customColors.containsKey(nodeId)) {
            // The attribute node contains all the attributes of the element specified in the markup
            // Even those which aren't valid for the element
            AttributeNode attributeNode = element.attributeNode();
            DynamicAWTSvgPaint dynamicColor = customColors.get(nodeId);

            // This assumed that the fill attribute is a color and not a gradient or pattern.
            Color color = attributeNode.getColor("fill");

            dynamicColor.setColor(color);

            // This can be anything as long as it's unique
            String uniqueIdForDynamicColor = UUID.randomUUID().toString();

            // Register the dynamic color as a custom element
            element.registerNamedElement(uniqueIdForDynamicColor, dynamicColor);

            // Refer to the custom element as the fill attribute
            attributeNode.attributes().put("fill", uniqueIdForDynamicColor);

            // Note: This class can easily be adapted to also support changing the stroke color.
            // With a bit more work it could also support changing the color of gradients and patterns.
        }
    }
}

class DynamicAWTSvgPaint implements SimplePaintSVGPaint {

    private @NotNull Color color;

    DynamicAWTSvgPaint(@NotNull Color color) {
        this.color = color;
    }

    public void setColor(@NotNull Color color) {
        this.color = color;
    }

    public @NotNull Color color() {
        return color;
    }

    @Override
    public @NotNull Paint paint() {
        return color;
    }
}

DynamicAWTSvgPaint dynamicColor = processor.customColorForId("myRect");

SVGPanel panel = new SVGPanel(document);
JButton button = new JButton("Change color");
button.addActionListener(e -> {
    Color newColor = JColorChooser.showDialog(panel, "Choose a color", dynamicColor.color());
    if (newColor != null) {
        dynamicColor.setColor(newColor);
        // Make sure to repaint the panel to see the changes
        panel.repaint();
    }
});
JPanel content = new JPanel(new BorderLayout());
content.add(panel, BorderLayout.CENTER);
content.add(button, BorderLayout.SOUTH);
frame.setContentPane(content);