When it comes to painting, there is only one thing you need to know in order to create art that is revered around the land...KEEP IT SIMPLE, STUPID!
find a colour scheme that you like, is quick to paint (for those swarms of cannon-fodder guardians) but is easily embellished (for those few special characters and, or course, spirit walkers!)
keep this in mind, and you will never go wrong.
For example. Pick Sky blue and black for your army colours. Great. Now on you character's cloak, you could still use the blue and black, but maybe set it up so that there is an Eldar cityscape stretching up through sky blue to black night, complete with twinkling stars.....
be creative!
Courtesy of CorsairRouge!