if they are RED then they are 4+ levels higher than you
if they are YELLOW they are 1-3 levels higher than you
if they are WHITE they are the same level as you
if they are DARK BLUE they are 1-5 levels lower than you
if they are LIGHT BLUE they are 6-10 levels lower than you
if they are GREEN they are 11-15 levels lower than you (you gain a very small amount of xp from green con mobs)
if they are GRAY they are 16 or lower than you and you wont gain XP from their death.
I believe this is true in most cases. However it also seems like the lower range cons are not set into stone. The higher level you are the larger the range of levels (lower that you) will be for mobs to give you EXP.
So RED, YELLOW, and WHITE are constant.
DARK BLUE, LIGHT BLUE, GREEN, and GRAY are a percentage.
I found more info:
DARK BLUE 94 - 99% of your level
LIGHT BLUE 82 - 93% of your level
GREEN 71 - 81% of your level
GRAY Lower than 70% of your level