I found this on DBG forum, this is from Dzarn

To make some sense of those numbers here is a chart that defines how much 'item exp' you gain per kill. (Basically, the amount of 'item exp' you gain per kill is based on your group size and the mob's level compared to yours.)

Grey cons at all group sizes yield 0 points of item exp.

Solo
Green: 20
Light Blue: 30
Dark Blue: 71
White: 111
Yellow: 120
Red: 131

Duo
Green: 12
Light Blue: 18
Dark Blue: 42
White: 66
Yellow: 72
Red: 84

Trio
Green: 9
Light Blue: 14
Dark Blue: 33
White: 53
Yellow: 57
Red: 66

Four People
Green: 8
Light Blue: 12
Dark Blue: 29
White: 45
Yellow: 48
Red: 57

Five or Six People
Green: 8
Light Blue: 11
Dark Blue: 26
White: 41
Yellow: 44
Red: 51
Think it may be helpful for us who work on evolving the items.