Stitch repeat in blanket: Multiples of 12 + 1 stitch.
Following the explanation above, change colours. Crochet six dc in the same stitch using your new colour. Don’t finish your last dc, but stop before the final yarn over (photo 1). Repeat the instructions for the colour change and complete the dc in the new colour (photo 2). Remove your hook from the loop, insert your loop from front to back through the first dc (photo 3) and grab your loop again. Pull this loop through the first dc, so the popcorn closes itself (photo 4). Your back will look like in photo 5. Crochet the next stitches over the thread of the other colour, so you can grab it again when changing colours for the upcoming popcorn (photo 6).
You will keep seeing bits of coloured yarn through the stitches. This is to be expected because it’s a deep purple yarn worked in white hdc’s. Don’t worry; you’ll barely notice this in your blanket. If it bothers you, cut your coloured yarn at each popcorn and re-join for a new popcorn.