The specific color is not relevant, they are different for each monster. When the 'glow' occurs, however, it means that it is it's turn to attack. Note the glow will occur ONLY if the monster has a valid attack. Since only the first monster group can use a melee attack, if the monster has only melee attacks, you will never see it glow except when it is the first monster group. This also means that if you see a monster in the back glowing then that monster MUST have at least 1 ranged attack. Note that the monster glows when it is his turn to attack, not necessarily when it's attack succeeds.
The bottom line is that if you see monsters in the back glowing, you had better check their stats to see what they might be up to, because some of the monster's ranged attacks can be pretty devastating.