Since you are using basic colors for your balls I would use a different basic color for the thread.

zigzag with varigated thread is my vote

Zig zag and match thread with fabric. It will look good.

This is just what we need to get us out of working on winter stuff. I think any color that you would like to do will be fine. As for me I would do a very tight zig zag in black or white. Marvel

Varigated thread in primary colors for all of it, in a satin stitch.

Love your bright colors. ZigZag sounds good to me also.
Good luck, Kutnso

I would use a zigzag and invisible thread also.

I vote variegated and zigzag. Have fun.

I have started using a small zigzag (.75 x .75) for my applique. I have had a lot of fellow quilters really like it as you don't even notice the stitching! Take some scraps and do some test stitches.
Diane/Wyoming

I would do a zigzag stitch and match thread for each color.