There's a great article in the most recent issue of Interweave Knits (p. 24), all about color blending! I was fascinated to stumble upon this article, because what they term "optical mixing" is precisely what we're doing here when we combine different input yarns to achieve your custom blend.
What they recommend in the article is holding two strands of yarn together and broaden your color spectrum by mixing the two. They use six different colors, which cover the range of the color wheel, and show swatches of the 21 different permutations you can achieve with this experiment. They show how different combinations can mix to form a new intense color, muddle to form a neutral color, and even how variegated strands can play into the mix.
At Yarnia you don't have to bother with holding the strands together, as they're wound together on a single cone for you, but the concepts in this article surrounding hue, saturation, and value can help a great deal in choosing the colors that will go into your custom blend. Of course the best way to make sure your yarn will turn out the way you're imagining is to try swatching up a little bit before we wind (and you're welcome to have a seat in the big cozy chair and take your pick from the vase of needles to do this!).

This article came at just the right time for me--this week I've been mulling over what yarn I want to use for a tank top I'm starting, and while I knew I wanted it to be some sort of green, I couldn't find anything that was working. This article inspired me to throw in a yellow, which hadn't occurred to me, but seemed worth a shot, as it is adjacent to green on the color wheel. In fact I tried not just a yellow, but what, on its own, seems to be a fairly obtrusive shade of yellow.
After working up a little swatch of it I was amazed to find that the mossy green brought the intensity of the yellow down, while the same yellow made the apple green look a little grassier. I totally love the blend, and somehow the rayon/wool/nylon combo makes the yarn feel as slippery and soft as the bamboo the pattern calls for!

We'll have a copy of the article out for reference so take a look the next time you come in...