Hey Simon,
I'll take a crack at your post. When chosing a colour scheme for Eldar, or any army, the first two things you should consider are 1) The look and feel you want to achieve for your army, and 2) Your overall ability as a painter to achieve the desired scheme.
That said, Eldar, along with Chaos, have an advantage over just about every 40K race in that they are well suited and open to a diverse array of color schemes. They work just as well with bright yellow and orange as they do with black and metallics. With that in mind, the first thing you should do is decide upon a two or three color scheme with, may I suggest, a suitably contrasted scheme that draws the eye.
This is often easiest to acheive with a dark color contrasted by a lighter or vibrant color, for example:
Black and Bone
Black and Ice Blue
Black and Yellow
Deep Purple/Violet with Yellow or Ice Blue
Black, Bone, & Ice Blue
Black, Violet, and Bone
etc...
Mixing and matching different darker and lighter colors with lighter one.
Another thing you have to consider is what effects you may use while painting to acheive your desired scheme. Rather than slapping down red on the helmets, then blue on the bodies, you could think about blending colours over certain parts of model. This can end up making the painting process complicated and longer, but will generally produce a more exciting scheme. Again some examples for your guardians:
Black bodies with red helmets that edge-out (blend) into blue at their extreme ends/edges.
Dark green bodies that blend into white at there edges
Black bodies and helmets that blend into red or blue at their extreme edges
and so on...
I realise I haven't provided a definitive answer to your question, but that was never truly my intent (I'm a butt-head that way). I think most will agree that the best scheme will be the one you choose and from there numerous great painters are available on these boards to help you acheive the effect and scheme your looking for.
Good luck and have fun,
Pauk777