Jeanharville, you are very discerning in how you've described the problem of choosing colors!
First, you are correct that all computer monitors display colors differently. You might know, for instance, that your screen shows things to be more vivid than they actually are. And you'll take this into account when you're looking at that screen.
The second thing is that those of us who sell online generally use the pictures supplied to us by the fabric manufacturers. And these pictures might - or might not! - be "color correct" compared to the actual bolts of fabric. Whether an e-tailer actually looks at the bolt when it comes through the door to see how accurate the picture is depends on how hard that e-tailer wants to work. Frankly, it takes quite a bit of time to look at each bolt and "fix" the pictures. I *do* take the time to do this; in fact, it was exactly what I was working on when I got distracted and decided to make a quick visit to the quiltingboard! As to what other stores do, I think that e-Quilter scans the fabric themselves (because I do remember reading a newsletter from them saying "our scanners are very busy this week"). Honestly, I doubt that most places bother to check the pictures, especially discount e-tailers. It's just too time-consuming.
I'm not sure how the "color pickers" work, where you pick a color out of a rainbow palette and then the computer pulls up every fabric with that color in it. But even if the computer could do this *perfectly*, you would still be left with your third issue, which is that different fabrics have different visual textures, even if they have the same colors. So which ones go together? Gee... this kind of requires an actual human to do the choosing, doesn't it? Like ckcowl said, this is where a store's customer service comes in. We get calls quite often from customers who ask us to put several bolts next to each other and tell them if the fabrics work well, or if we can recommend something else. And we're always happy to do it.
This is where... ummm.... size matters. If the customer service department you're calling is in an office that's not near the warehouse where the fabrics are kept, they're not going to be able to help you coordinate your colors, are they? It's possible the customer service folks have never even laid eyes on the individual fabrics. Whereas a small company has the fabrics right in the next room, and they're going to be familiar with each one, and they will take the time to help you choose fabrics that you will be happy with, and will enjoy working with. These little e-tailers - just like your little LQS - are giving you something that you just can't get from the giant discount stores. They are giving you individual attention and help. And yes, they do have to charge a little extra for this service, because it takes time. But when you think of all the time that you spend creating a beautiful quilt... and the enjoyment that comes from the creative process... you really want to have the best combination of fabrics. And when you're buying long-distance, it's nice to have a little help choosing.