Shoutbox

I think what you are asking is to have a toast message display when someone comes online, but have the toast display the custom nickname with colours that you have already set up? If that is what you want, this can be done with Plus all by itself.

The first thing to do is to disable the default MSN toast messages. This can be done by going into the messenger toolbar.
Tools > Options > Alerts & Sounds. Uncheck the box labeled "Display alerts when contacts come online"

The next step is to enable the Plus toast messages. Navigate to the Plus Preferences > Customize > Events Notifications. We are going to add an event.

You will probably want to add this to be for all contacts, but there is an option available on this new window that comes up to specify only certain contacts for the toast message. Click Next.

On the next page, you will be able to select which event you want this notification you are creating will be for. You want the first option, "User has signed in" Next. You will want to enable the popup message option and select "Generate automatic notification". This will display the custom nickname you have set up within MSN, and state what the user is doing (signing in). Optionally, you can also enable a sound notification. If you do, the best bet would be to use the default MSN ding sound, located at C:\Program Files\MSN Messenger\online.wma

Now, for custom names. This setup will automatically use the custom named within MSN that you have set. Good news! the custom names that you create do support the colour code tags that Plus uses.

Hopefully this will be what you want, and if you need clarification on anything, please do respond.