In Battlemaster, there are several types of messages you can send, with various different contexts to them.
A Letter is just that. It is in the style of a letter, with varying degrees of formality. It should look something like this:
Greetings, Lady <Whatsherface - these old letters do tend to get messed up you know.>!
I would be honoured if you would grace us with your presence at the royal court this evening.
A letter has a normal, black background.
A Roleplay is where you, as a player, describe what your character did, what happened - indirect speech from a 3rd person perspective, preferably from an IC witness. Example:
Sir Lancelot faced Sir Galahad in the duelling ring. Glaring out of mutual dislike, they lunged at each other, managing to wound themselves lightly before retiring, to the disappointment of the crowd.
A roleplay has a gold-brown background.
The General asked for scout reports? The Marshal wants to know your unit status? You're making a general observation to the realm? Send a report.
Mostly this is for the menial day to day things that happen to your unit, or in wartime.
Reports have a blue background.
You're asking for cash? The General wants those scout reports delivered without shouting? Probably use this messaging option.
Requests are, as the name suggests, used for requesting something.
Requests have a green background.
You should probably get a few sets of these per day, They will usually contain relevant information on where to move, what settings to use, whether to loot, takeover, demolish, dig in or give any of the other myriad Orders you personally have to to your unit.
Standing Orders stay at the top of the message page and indicate what the entire army should be doing for an extended period of time. A common standing order might tell you what region to rally at, or what settings the army should be using for the next few turns. Only a marshal, vice-marshal, or general can edit the standing orders.
Orders have a dark red background. Standing Orders have a bright red background.
This is for anything you, as a player, wish to say, rather than your character. You might use it, for example, for saying that you will be away over the weekend, or that you have been abducted by oddly mathematical obsessed aliens who will only allow you to long on when the date does not have a prime number in it.
OOC messages have a grey/white background.