Relative xp - how does it work?

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I'm not sure and that's why I'm posting this.

The way I've always understood this is that it is best to fight dark blue mobs as any higher than that and they will beat through your AC and resist your spells. Makes sense. On the other side of this you are advised to stay away from light blue mobs because they don't give much xp. The end result is that you are best off fighting as low level dark blue mobs you can find (again, ideally just before they go light blue). My assumption, given that I do not have a fancy UI that quantifies experience points is that there is some kind of experience modifier that is applied to each "con" with a positive being applied to whites, yellows and reds, none to blue and negative to light blue. If experience is applied linearly, this theory seems to flesh out although my theoretical application of the penalty/reward might be different.

The question I have is - can anyone confirm that there is an experience penalty applied to lb mobs or have a number they can post to verify this? In addition, how would a modifier work within a group where one member may con the enemy dark blue to your light blue con of the same mob? Is the penalty removed? How about white - red? Same thing?

Thanks for your help p99!!