ColourCommand.java

public class ColourCommand implements CommandExecutor
{
    @Override
    public boolean onCommand(CommandSender sender, Command cmd, String label, String[] args)
    {
        //Say for example we are making a command like this /colour <type>

        //First we check if the sender is a player (sender can also be the server console or a command block).
        if (!(sender instanceof Player))
            return false;

        //You have to make sure you checking there are the right number of arguments. In this case we are only looking for one, "<type>".
        if (args.length == 1)
        {
            switch (args[0].toLowerCase())
            {
                case "blue":
                    //The command must be /colour blue
                    break;
                case "red":
                    //The command must be /colour red
                    break;
                default:
                    //The first argument is not a colour
                    return false;
            }
        }

        //Return true for successful commands, and vice versa.
        return true;
    }
}