After this post I made another practice square, see attached picture. This one came out better than previous efforts but it is still not quite right at the start of the diamond. (It looks okay in the picture but not in real life.)
I found that controlling tension in the just completed knitted stitches at the color changes is critical, though quite difficult to do. Too tight and the stitch disappears, too loose and it bulges out of proportion with its neighbors. You have to keep tweaking each color yarn until it stays just right.
The other thing I learned was about tying the knot at the beginning of the new color. At first I was leaving it loose and tying later which was not possible with nothing to tie to and my attempts created a bulky bulging mess. A tight and tidy square knot with the new and old colors after completing the very first stitch worked well.
This argyle is a true diamond, not a square which requires a little more thinking. I made color changes on the WS/purl side. I could not figure out how to do it on the RS though surely it is possible?