Online purchasing.....
 

Are you on Ravelry? You can see what people have knit from most yarns and see how it looks. They also comment about the yarn and that has helped me a lot!

I'm still not good at imagining what yarn in a picture online will look & act like in person, but here's what helped me.

I read this forum and when Ravelry came along, explored that...and listened to podcasts and such.

I heard and read the names of many yarns. Then I had a chance to go to a local yarn store (LYS), and lo and behold, there I found labels on yarns and found myself saying, "Oh! So THAT's Noro Kureyon!" or "So THAT's what Cascade 220 is like!"

I mainly just started paying attention...I fondled yarn at the yarn store, and noticed the softness of bamboo and whatever.

I saw some Rowan Kidsilk Haze and wondered why anyone in the world would want to knit with it, though I hear it raved about now and again.

So..the more you explore, the more you will learn. If you're like me and a visual person, seeing (and feeling) in person will help. I try to buy a little something everytime I go to the yarn store, but really I'm just learning about what I see in patterns and magazines and on Ravelry and here.

In the meantime, pay attention to gauge. I now can realize without even looking that a yarn that gets 3.5 stitches to the inch on size 10 needles is going to be a chunky yarn---and one that gets 7 stitches to the inch on size 3's is a pretty skinny little yarn...just from looking at ball bands and paying a little attention to patterns and such.

I did ask a lot of questions in the beginning, but really, I found that a lot just has to be learned by experience (or math, but I try to avoid that whenever possible! :))

I was a bit wary of buying from online at first but after a bit more experience with yarn and the discovery of Ravelry I have made a few purchases from online and the yarns have mainly been what I expected. It's important to remember that colours will appear slightly different from onscreen to real life and check the details of the yarn!! Some websites don't always give you the full details on the yarn so I often check it up on Ravelry and Ravelry is also good for the points that Jan already mentioned.

You can also get color cards from many of the online places. Although the pieces of yarn are tiny, they will give you even more of an idea of what you are ordering after checking out Ravelry and Yarndex.

Another thing that I do is I check out pics that people on Ravelry have put up of the particular color of the yarn I want to buy (including projects made with it). If it's something more popular like Malabrigo, Noro, or anything from Knit Picks, you can get a feel for the yarn by simply seeing it in many different lights. Whenever I'm in doubt as to whether or not I'll like the color, I check Ravelry.

The only problem with this is that every monitor can see colors a little differently and photographing colors can be way off. I kind of know what I'm doing with photos, but I still have problems with some colors. It's a good way to see how the colors stripe or whatever though.

I've bought a lot of knit-picks color cards, espescially the Palette and Telemark yarns for colorwork. It helps to plan projects. And while it's true the samples are small, it will be obvious if a yarn is say, a fingering weight when you were looking for a worsted, as in your example.

I'm probably going to get slapped for this but I also go into my local yarn shop and feel the yarn I see online. Now it doesn't work for KnitPicks but when looking at the bigger names like Cascade and those types, the local shops carry it.

Unfortunately their prices are so much higher that I can rarely afford to purchase locally. I know that's a bad thing but I am on a tight budget and need to use my dollars wisely and make them stretch. If I find something I just want to try, I may buy one skein now and again.

That's one reason I look at a bunch of diferent photos. That way, even if every one of them is a bit off, as a whole they give me a much better idea of what I'm getting into :) But color cards are always best.