Brent, you get your own full on response post for asking questions and restoring my faith in the assumption that my hobby content actually gets read. Sorry for the delay as I think I've figured out that this question is in regards to your BoLScon Footdar extravaganza.
This is fantastic advice. I've got an army in the works now, with just the base purple on. I'm sort of stalled out on what direction to go, but I'll premix some colors first. As an aside, what works well with purple for a dark army? Think foundation paint highlighted up a stage. I'm thinking of Warlock Purple and some form of blue for the accents, but I'm not sure if that's enough contrast. Sorry to bug you with this - Brent
No need to apologize for asking questions, my man . Sharing, like swearing, is caring.
Before I get into my advice on purple, I'd just like to say that the problem with your Storm Raven pretty much begins and ends with the color you painted it. It should have been red. Other than that, I think its pretty good - its unique, actually looks like something that flies, carries dudes and shoots guns. But with nary a lick of red paint or Blood Angels icon to be seen, there isn't much to draw people's attention away from its shape.
Don't feel too bad about bad color scheme choices. We've all done it.*
Now, moving forward from my words of encouragement/brilliant segue, we're at the part of the post where I answer your question.
The first piece of advice for you is to do a test model - it doesn't even have to be Eldar. Just something you can do some of what I call 'sketches' to see what your color scheme would look like. You're not even aiming for a finished product or high standards - you're simply roughing out some colors to see what they look like together. I find this is great help when I'm messing around with new ideas. Its a relatively quick and low impact way to make sure that your army is on the fast track to coolsville and not clown college.
Paint the test model... don't look at it for a bit, then (later) look at it up close, far away and under different kinds of lights. Show your friends. Make sure you're happy with the colors before you commit.
As far as the color selection goes - If you're in love with the idea of purple/blue accents for the hormagaunt purple, then pick one or the other. I think doing both an additional purple and a blue will muddy the minature. I.E.: make it look indistinct at distance. So pick one.
If you go this route, you may want to consider some of the science Sandwyrm dropped on us recently. Part III of his, frankly, amazing series deals with different kinds of color harmonies and gives us some some very interesting things to consider. If you want to go with a third color, you may want to consider a split-complement kind of thing. I'd keep the third color very small areas and for picking out small details. With any paint scheme, you don't want to overload on main colors.
Just as a quick note, while I deeply respect the kind of information Sandwyrm has presented, its the kind of thing that can quickly drive you mad if you focus on it too much and hold it up as some sort of set of rules. Its just another tool in the toolbox. An extremely useful and powerful too, but remember - there aren't to many hard and fast rules when it comes to aesthetics.
Now, in regards to a split compliment - if you end up with two purples or even a blue and a purple I think you could do some pretty cool things with some of the complimentary colors. I think yellow could be good. So would a light green, but I like this idea the least. Better still, I think some gold details would look fantastic. For my money, a gold paint more on the yellow side of things would be best - something like the Formula P3 Brass Balls.
Moving beyond the colors you suggested, some more neutral colors could work wonders for accents - black has potential, but it might get lost in all the purple as they're both dark colors. Again, this is why test models are heroes. I think some grays may be a better option for this route.
Actually, back to gold for a second - I'm really thinking that no matter what you do, some gold details would look great - keep that in mind.
And there you have it Brent. Those are the kinds of things I would play with first. Just keep in mind that since painting is so subjective, that my advice is based largely on my opinions - so I may be dead wrong in someone else's eyes. Which is why test models are so important.
Above all else - no matter how good you painting skills are in a technical sense, crap color choice will always kill you.
*The much laughed at McWorld Jacks. Ugh.