On the multiplayer scoreboard, some number ranks display in colors (red, blue, green) as opposed to white (like me). Also there aren't three-digit numbers, but there are medals (wings?) that also come in colors. I can surmise that the wings represent level 100 and the colors represent "prestige-ing." I don't remember this from Gears at launch. What's the deal?
The coloured ranks were introduced as part of the Fenix Rising DLC.
The Re-Up System allows players that have reached level 100 "re-up" for another tour of duty, which resets experience and rank, however, other progress in the game will be saved.
Players can "re-up" up to three times, the different colours represent how many times a player has done this. The levels are as follows (in ascending order of prestige):
There are also achievements associated with choosing to re-up (Time to Re-up! and Again!), as well as unique weapon skins.
More information can be found on the Gears of War wikia page, if you're interested.