Many of the Mafia commands involve targeting a specific player, which you can generally do in a couple of different ways. Many night actions simply allow you to to click a box next to the player. However, there are other actions which require typing a command and either including the player's number or color. For example, to send a private message to player 4, you would type either "-pm 4 [Message]" or "-pm purple [Message].
Below is a list of the 15 player slots in Mafia and their respective colors for your reference. You may find this page helpful, especially if you do not have a lot of time to type your command in before the day or night is over.
|8||Light Pink||Light Pink|
|11||Dark Green||Dark Green|
|13||Light Green||Light Green|