Creates RBG glow below a player in minecraft using the Paper API.

PlayerMoveListener.java

package me.delta.mc.wand.listeners;

import org.bukkit.Bukkit;
import org.bukkit.Color;
import org.bukkit.Location;
import org.bukkit.Material;
import org.bukkit.block.Block;
import org.bukkit.entity.BlockDisplay;
import org.bukkit.entity.Display;
import org.bukkit.entity.Player;
import org.bukkit.event.EventHandler;
import org.bukkit.event.Listener;
import org.bukkit.event.player.PlayerMoveEvent;
import org.bukkit.plugin.Plugin;
import org.bukkit.util.Transformation;
import org.joml.AxisAngle4f;
import org.joml.Vector3f;

import java.util.HashMap;
import java.util.Map;
import java.util.concurrent.atomic.AtomicInteger;


public class PlayerMoveListener implements Listener {

    private final Map<Location, BlockDisplay> displays = new HashMap<>();

    private final Plugin plugin;

    public PlayerMoveListener(Plugin plugin) {
        this.plugin = plugin;
        this.applyRBG();
    }

    @EventHandler
    public void onPlayerMove(PlayerMoveEvent event) {

        if (!event.hasChangedBlock()) return;

        if (this.displays.containsKey(event.getFrom().getBlock().getLocation().subtract(0, 1, 0))) {
            this.displays.get(event.getFrom().getBlock().getLocation().subtract(0, 1, 0)).remove();
            this.displays.remove(event.getFrom().getBlock().getLocation().subtract(0, 1, 0));
        }

        Block block = event.getTo().getBlock().getLocation().subtract(0, 1, 0).getBlock();

        if (!block.isSolid()) return;
        if (this.displays.containsKey(block.getLocation())) return;

        this.spawnDisplay(block.getLocation(), event.getPlayer());
    }

    private void spawnDisplay(Location location, Player player) {

        location.getWorld().spawn(location, BlockDisplay.class, blockDisplay -> {
            blockDisplay.setBlock(Material.WHITE_STAINED_GLASS.createBlockData());
            blockDisplay.setTransformation(new Transformation(new Vector3f(0, 0.99F, 0), new AxisAngle4f(), new Vector3f(1, 0, 1), new AxisAngle4f()));
            blockDisplay.setInvulnerable(true);
            blockDisplay.setSilent(true);
            blockDisplay.setGravity(false);
            blockDisplay.setBrightness(new Display.Brightness(15, 15));
            blockDisplay.setShadowStrength(0);
            blockDisplay.setVisibleByDefault(false);
            player.showEntity(this.plugin, blockDisplay);
            blockDisplay.setShadowRadius(0);
            this.displays.put(location, blockDisplay);
        });
    }


    private void applyRBG() {

        AtomicInteger r = new AtomicInteger(255);
        AtomicInteger g = new AtomicInteger();
        AtomicInteger b = new AtomicInteger();

        Bukkit.getScheduler().runTaskTimer(this.plugin, () -> {

            if (r.get() > 0 && b.get() == 0) {
                r.getAndAdd(-15);
                g.getAndAdd(15);
            }
            if (g.get() > 0 && r.get() == 0) {
                g.getAndAdd(-15);
                b.getAndAdd(15);
            }
            if (b.get() > 0 && g.get() == 0) {
                r.getAndAdd(15);
                b.getAndAdd(-15);
            }

            this.displays.values().forEach(blockDisplay -> {
                blockDisplay.setGlowColorOverride(Color.fromRGB(r.get(), g.get(), b.get()));
                blockDisplay.setGlowing(true);
            });

        }, 0, 1);
    }


}