Untitled

import javax.imageio.ImageIO;
import java.awt.*;
import java.awt.image.BufferedImage;
import java.io.File;
import java.io.FileInputStream;
import java.io.IOException;

public class TransparentPNG {

    public static void main(String[] args) throws IOException {

        BufferedImage bi = ImageIO.read(new FileInputStream(new File("/home/oxodao/Small-mario.png")));

        for (int y = 0; y < bi.getHeight(); y++) {
            for (int x = 0; x < bi.getWidth(); x++) {
                Color c = new Color(bi.getRGB(x, y), true);

                if (c.getAlpha() == 0)
                    bi.setRGB(x, y, (new Color(13, 160, 0)).getRGB());
            }
        }


        ImageIO.write(bi, "png", new File("/home/oxodao/small_green.png"));
    }
}