If I were going to cast on, I would get both colors and cast on one stitch with one color, then one stitch with the other, repeated across.
If your finished piece will be a motif that is 20 stitches wide, you’ll need to cast on a total of 40 stitches (20 for the front, 20 for the back).
If you find it confusing to cast on alternating yarns, you can make it easier by doing the following. Let’s say, you need 20 stitches of each color. You’ll need 3 needles in the size required for your project. Cast on 20 stitches in color A onto one needle. Cast on 20 stitches in color B onto a different needle.
Get your third needle. Slip one stitch from one needle, then one stitch from the other, back and forth until all 40 stitches are on a single needle.
Hint: It’ll be much easier if you do this loosly.
Amy or someone else, may have some other suggestions.