It means your current Al level is higher than the creature (when they are green). You get more xp from yellow and red creatures. You still will get xp from green ones though.
Unless this is a new change, you don't get more XP based on the color-coding of mobs. Your Adventure Level determines the color of mob names. As the colors go from Red to Green the mobs get easier to hit. Basically, green coded mobs are easier to hit than red.
I've thought for a while now that Mobs color only affects its level compared to yours, just a quick indicator of "Will I struggle or not against these mobs", not affecting anything in the combat math at all, just being an indicator... Can someone confirm or tell me how wrong I am in thinking this?
Basically it's an indicator as to whether or not you're likely to survive having a go. Whole you will get the same xp for the same mob, a yellows XP will be more appropriate to your level than a greens.
I too would love to have confirmation that they would be easier to hit and perhaps they have a lesser chance to hit me?
With no death penalty now (just be sure to resurrect at an ankh), just explore and find out yourself!