Set a different armor color per observer.

ExampleMod.java

package com.comphenix.example;

import org.bukkit.Color;
import org.bukkit.event.Listener;
import org.bukkit.inventory.ItemStack;
import org.bukkit.inventory.meta.LeatherArmorMeta;
import org.bukkit.plugin.java.JavaPlugin;

import com.comphenix.protocol.Packets;
import com.comphenix.protocol.ProtocolLibrary;
import com.comphenix.protocol.ProtocolManager;
import com.comphenix.protocol.events.ConnectionSide;
import com.comphenix.protocol.events.PacketAdapter;
import com.comphenix.protocol.events.PacketContainer;
import com.comphenix.protocol.events.PacketEvent;

public class ExampleMod extends JavaPlugin implements Listener {
    private ProtocolManager manager;
 
    public void onLoad() {
        manager = ProtocolLibrary.getProtocolManager();
    }
 
    @Override
    public void onEnable() {
        manager.addPacketListener(new PacketAdapter(this, ConnectionSide.SERVER_SIDE,
                Packets.Server.ENTITY_EQUIPMENT) {
            @Override
            public void onPacketSending(PacketEvent event) {
                PacketContainer packet = event.getPacket();

                ItemStack stack = packet.getItemModifier().read(0);

                // Only modify leather armor
                if (stack != null && stack.getType().name().contains("LEATHER")) {

                    // The problem turned out to be that certain Minecraft functions update
                    // every player with the same packet for an equipment, whereas other
                    // methods update the equipment with a different packet per player.

                    // To fix this, we'll simply clone the packet before we modify it
                    packet = event.getPacket().deepClone();
                    event.setPacket(packet);
                    stack = packet.getItemModifier().read(0);

                    // Color that depends on the player's name
                    String recieverName = event.getPlayer().getName();
                    int color = recieverName.hashCode() & 0xFFFFFF;

                    // Update the color
                    LeatherArmorMeta meta = (LeatherArmorMeta) stack.getItemMeta();
                    meta.setColor(Color.fromBGR(color));
                    stack.setItemMeta(meta);
                }
            }
        });
    }
}

gud