That scarf looks really amazing, I hope you can figure out how to recreate it.

I had a thought... If you could get a plastic container and punch 5 holes in the lid to thread the different balls through, that might help? Not sure it would work, but it's an idea.

If you did 2 x 2 rib and changed colours between two knit stitches (or purl on the reverse) you might get a better join.