I started in the middle with one ball, loosely modelled on what Polly has going on here.
O.k. let's be honest, I bought one ball, and my friends were horrified I would want to make a baby blanket out of something so scratchy. Then I went back and bought two more balls and this thing grew firmly outside baby blanket dimensions. (It's too scratchy to be super cuddly round your neck, but perfectly acceptable to have across your lap over clothes.)
So I started in the middle with one ball and a loop and 4 granny chains with Michelle's 4.0mm hook. I haven't forgotten I need to get the hook back to you Michelle! And then I just grannied my way around, with two grannies in each corner, and joining each round so it didn't spiral.
As the blanket grew the colour rings got thinner, so I just joined in my extra balls at approx the right colour spot. I divided some of the balls up to get more colour transitions out of them. By the time I was finished I was crocheting 5 balls around, just joining them up as they butted up to each granny. This way I was able to keep the colour transitions approximately equal in size- it's not perfect but pretty close!
Then I used the bits of purple I had left to head around the edge, just to finish things off. (Also, I'm a purple bridesmaid.)
Finished photos tomorrow....