@boeing797, you are right on the performance/economics part.
But don't forget why (some) airlines are trying to design good-looking liveries. A good-looking plane and/or livery is remembered by people more easily then a boring looking one.
So it is all about the marketing aspect, I guess a psychiatrist can tell you more about how the human brain reacts to it.
To give an example, when the B7E7 project was presented two friends in my class (who are not into aviation) came over to me and told me that they saw pictures of the Boeing 7E7 and they thought it looked awesome. So they remembered it, and I never heard them about other aircraft that were presented (but those other aircraft didn't leave a remark in their heads).
One could translate that into -> passengers will more likely remember a flight when there is something distinctive about it (great looking aircraft/livery, great interior, great IFE, great service i.e.). And when an airline is positively remembered by passengers, they are very likely to choose them again, or to recommend them to other people.
Marketing can be a very strong instrument, do not underestimate that.
