Each element has a different set of energy levels. Use this fact to explain why the colour of the light emitted by an element in the gaseous stare is characteristic of the element?

Question

Ive read and researched Quantum physics for over a week now and i still don't understand this question.

Since this is not my area of expertise, I searched Google under the key words "energy levels spectrum" to get these possible sources:

(Broken Link Removed)
http://webbook.nist.gov/chemistry/polyatom/
http://science.howstuffworks.com/atom7.htm
http://online.cctt.org/physicslab/content/Phy1/lessonnotes/atomic/atomicmodelsandspectra.asp

I hope this helps. Thanks for asking.