EXPFollow

The sweet spot for single target encounters (and quest turn in) is given to be yellow or orange. Even con targets (white) are fine but will take about as long to kill and will yield less XP (then yellow/orange). Blue, lt blue and green are easier (faster) to kill but will yield far less XP.

However, depending on class and technique, you might be able to make a case for killing blue so much faster that it evens out (In most cases I don't think it will be a valid argument though!)

Needless to say, killing red targets is very, very difficult and you will likely be unable to lay a glove on anything that cons purple to you so regardless of their XP value they are not recommended!