As promised, here is the tale of how I got the colours to pool so nicely in my smaller Rainbow Cake hat. The pastel rainbow colours of the yarn (Madelinetosh's Pure Merino Worsted in 'Pocket Rainbow') practically demanded that I have a go at controlled colour-pooling, and I couldn't be happier with the way it worked out. :)
The first step, of course, was swatching. I cast on about 40 stitches, knit in seed stitch until I reached the start of a pink section of yarn, and then counted my stitches until I reached the same point in the colour cycle. I repeated this a few times to make sure my count was consistent (to within a stitch or two).
I discovered I was getting 70 stitches per colour repeat, in seed stitch on 4mm needles. The smaller size in my Rainbow Cake pattern-in-progress had a 72-stitch cast on, which I decided was close enough for the pooling to work.
The first time I cast on for the hat, I used smaller needles to get a nice tight, tidy ribbing section. This change of needle size caused the colours to line up in little rainbows instead of pooling, which looked really cool! But unfortunately the stitch pattern was obscured a little too much, and I decided to start over.
The second time, I used the same needle size throughout in order to get consistent 'stacked' colour pooling right from the cast on through to the crown decreases. This time the stitch pattern was clear, and the yarn complemented the design rather than muddying it.
While knitting the hat, I found I often needed to tink/un-knit part of a round to get the colour repeats to line up nicely. My tension definitely changes depending on how fast I'm knitting and how much attention I'm paying to it, and I found myself zooming through the cream sections to get to the next 'rainbow', only to discover I had far too much cream yarn leftover. At least I'm pretty good at tinking these days!
I was happy to see the colours begin to swirl when I started the crown decreases, making a little whirlpool around the pompom. :)
If you'd like to try deliberate colour pooling, my advice is to swatch carefully, stick to one needle size, and be prepared to do a lot of tinking/un-knitting if your tension is a little wonky like mine.
This Twist Collective article by Karla Stuebing goes really in-depth into how pooling works: The Art and Science of Planned Pooling.