Look at the code for JoinCommands by @PixelGuy75. The code is simple to understand(especially for you).
Try to add § + color ID in the message configuration Example: " §aHello player!" (The message should be green)
My idea was without editing the source code. I didn't try JoinCommands but I think that commands and messages can be edited from configuration file. I think editing source code is not needed for colored messages