ColourTab.java

public class ColourTab implements TabCompleter
{

    @Override
    public List<String> onTabComplete(CommandSender sender, Command command, String alias, String[] args)
    {
        //This is how to set up tab complete too
        if (args.length == 1)
        {
            String[] list = { "blue","red" };
            List<String> listFinal = new ArrayList<>();
            String al = args[0].toLowerCase();

            for (String c : list)
            {
                if (c.toLowerCase().startsWith(al))
                    listFinal.add(c);
            }

            return listFinal;
        }

        return null;
    }
}