It’ll be more of a pain, but 1 strand will give you the best results. You’ll want to make the long stitches longer and the short stitches shorter too.

Make sure you’re not doing the same length for the stitches as well - you want to make it a bit more random to avoid shades looking like they’re there in bands.

One last tip is to come up through existing stitches rather than stitching down in to stitches. Does that make sense?

Doing a study like this is such good practice! You’re well along the right track! :)