Lotus, that looks like a great blankie!
I've read the instructions. When you cast on, it is with the main color only. This makes sense if you look at the photo, as the border is only one color. You then work the preparation row still in the border color, doubling the number of stitches on that row. From there, you can continue on in one color only if you want to, or if you are using two colors, the row after the preparation row is where color two is brought into the project. Read down to Variation #2 and that's where it discusses this part.
Don't give up, that will be a warm squishy blanket.