As I am working on the master pattern for our granny square a-long (say that to the non-crocheter and watch their face), I thought we could talk about the yarn you'll need (thanks for asking about it Erin).
You can read about the original idea of the granny square project here.
If you already own a lot of yarn, I would encourage you to use this pattern as a stash buster if possible. That's what I'm doing. With no shortage of yarn around here (a hem, the above photo is the top of a basket that currently runs very deep), I thought it would be a good challenge to impose some color limitations on myself, maybe mixing in a few I don't really love with the favorite ones I always grab. Just like I realized in Denyse Schmidt's Improvisational Patchwork class last summer, taking away a lot of the choice can yield some really fun, unexpected results.
And I figure that emptying my yarn basket is a good excuse to buy more.
The original pattern that inspired the idea for this project uses 12 different colors. That's a good starting number as you gather your yarn colors for this project. Granny squares use lots of colors, but not a ton of yarn. If you are buying yarn, 1 skein per color is probably sufficient. But there are exceptions! For example, one of the colors I will use often will be cream, and I will most likely buy 2 or 3 skeins of that color if I don't have enough.
Now, the yarn size. The original inspiration pattern specified worsted weight yarn, and our pattern will use worsted as well. This is considered a Category 4, Medium weight yarn. Cotton, Acrylic, Wool... you choose the content, but hopefully you have some stash yarn that falls into the worsted weight category. I will use Cascade 220 and Cascade 220 Superwash, both 100% Wool and available in a rainbow of colors. I have also been using Debbie Stoller's Stitch Nation "Full O'Sheep" 100% Wool. Available at Joann's and Michael's, its per skein price is less than Cascade, but there aren't as many colors available. If you have a finer yarn you want to mix in, you can still use it! Just be selective about where it goes, spread it around the afghan and know that the effect of that area will be lacier and more open (you will still use the same hook). A thicker yarn, is not going to be fun to crochet with.
Hook size- I use Clover Soft Touch Crochet Hooks. I love them! I learned to make granny squares using a size 5.0mm/H and I now use a smaller size, a 4.00mm/G size because I like my squares to be tight. Its a personal choice what you use, any hook compatible with worsted weight yarn is perfect fine.
One last thing, because this is a sampler afghan, with lots of different granny squares, using lots of different colors, it might feel daunting to choose colors randomly (or not) as you begin. If you, like me, enjoy a few rules, make some for yourself.
Lightest color always starts in the center and gets darker with each round.
Blue and Pink yarns never sit side by side (that's one of mine)
Always end largest rounds with charcoal heather.
Use cream in every granny square.
It's is entirely up to you, but something to keep in mind when shopping for yarn.
OK, fellow crocheters! Go forth and choose your yarn rainbow!