I agree. Stitching in the ditch will add dimension to the ball and not detract from the design like a top of the seam would. Great idea.

I think I would use varigated thread in primary colors for all of it.

I know this is too late but I would use black to really make the colors stand out. I was thinking zig zag in the same color but you would have to use 2 rows for each seam of zigzag to do that. And I like black with all of those bright colors. Maybe look a bit like stained glass I am hoping for.
JeanneZ

If your machine has decorative stitches this would be a fun place to try them out. Like the verigated thread but if you use monofiliment be sure to loosen your top tension.

I would use a bright rainbow varigated thread and use a tight zig-zag stich. You can use either a off white or a light grey for the bobbin. I would also recommend a top stich needle to do the satin stich with.

I would also do this.

I love this quilt. Did you design it yourself, or did you have a pattern? I would love to make one. I am not very exact when creating on my own, so any suggestions would be appreciated. Also, will there be a binding, and what is the back like?

Zig-zag with varigated thread on top and in bobbin.

This is going to turn out neat, no matter what color you use. I like the idea of black SID for the ball.

Wow, I didn't think this would as interesting a post as it has been - look at all the different suggestions! LOL* I never would have considered what some the other posters mentioned, how neat! Now I'm inspired. ;)

If it were me I would probaby go with a zig zag but with invisible thread. I don't care for the wide zig zag on my machine, I usually drop it down in size a little bit, it seems to look neater that way, at least with my machine. I love the idea of going in the ditch down the lines with a straight stitch for each of the colors. You could be sure then it would be nice and secure, and it would add a little dimension to your beach ball too.