Roy's name is one Arakawa hasn't put little thought on, that's for sure. Both his given name and his surname have a very particular meaning, that goes esceptionaly well with Roy's personality and purpose in the series and which has a very particular symbolism.

Let's start with his first name, ROY. A name which at first glance would seem a simple English name, with no purpose, it's exactly the contrary. From a French-English dictionary:

Indeed, the name given to the man whose crusade consists in becoming the leader of Amestris is in fact king in French. At this time, we have no way of knowing if Roy will reach his goal someday, but what's clear is that Arakawa has Roy's quest very present in her mind. This is especially significant if we consider that the current Amestris Fuhrer's name is no other than King Bradley.

Names with the exact same meaning, yet totally different outcomes. Bradley is a man who has betrayed his country in the worst way possible, even if could be argued just how forced he was to do so. He is not working for his people, but rather using them for his own plans (or rather, Father's plans), up to the point where he would order his army to eliminate an entire race. Roy, on the other hand, is Amestris' greatest hope, even if they don't know it yet. He's willing to commit his entire life to make his country a better place and to stop all injustice. He's willing to sacrifice himself for it.

Roy's surname, Mustang, is part of a recurring trend from Arakawa. A good number of the military personnel's names and/or surnames have their origin in war aircrafts and other related stuff. That's the case in Hawkeye, Havoc or Fuery, to name a few. Mustang is also one of them.

The P-51 Mustang was a successful long range one-man fighter aircraft which entered service in the middle years of World War II, and its first flight in history was in 1941. The Mustang's combat record is generally considered to consist of 4950 air kills, 4131 ground kills and 230 V-1 kills. This airplane is considered by many to be one of the greatest war aircrafts ever to be designed, and some have even go as far as to say that it was the fighter that "won" the war.

Whatever version you decide to choose, it's obvious that Arakawa chose the name "Mustang" very carefully, as if to imply Roy has the capability and will to put up a good fight for Bradley, regardless of the outcome it may have.

And just in case you were wondering, in the Japanese version of FMA, Roy Mustang is written entirely with katakana, which is the alphabet used for western words (and also used by children before they learn hiragana and kanji):

Now, speaking of how people address to him... the most common way in addressing Roy is "Colonel" ("Taisa" in Japanese) or "Colonel Mustang", as it is his rank. It's used by Hawkeye and all of his subordinates, as well as Hughes, Armstrong and the Elric Brothers. However, he's also been called "Mustang-kun" (the suffix "-kun" is used for younger men or boys) by his superiors, like General Grumman. A younger Hawkeye used to call him "Mustang-san" before she entered the military. The only ones that have ever addressed him by his first name, "Roy", are Hughes and his late alchemy teacher, Hawkeye's father. At least for now.