The announcements.yml file is where all of MobArena's announcements and messages are stored. You can freely edit this file however you see fit, and color codes are supported. To add color to a message, use the &-character followed by a valid color code. You can find a list of valid codes right here, and you can find examples in the default file.
Note that some of the announcements take a variable, represented in the message by a %-character. You can leave this character out if you don't want the variable in the message. It is not possible to add variables to announcements that don't take them by default, and announcements that take one variable cannot take more than that one variable.
If you don't want to see a specific announcement, you have to set its value to '' (note: two single-quotes, not one double-quote). This will cause MobArena to ignore the announcement. Note that simply removing the node will not work, as MobArena forcefully adds it back in.