Is it French? I don't speak the language. Maybe it means shades of a color hue, like shades of purple or shades of blue. Maybe an artist term.

I believe:to the letter ton-sur-ton( french) is tinge on tinge, or the use of the same colour in one design. there can be a small difference in colour brightness but in principal it's the use of the same colour, sometimes in different nuances.
I'm not Mops as you can see but this is what I think it is

Sounds like - tone on tone - to me! But I don't speak French.

It's been a long time since I used my French, but this in tone on tone - which to me means using same color, or within color family. Hope I am correct.

It's a french expression. It means, in embroidery, that you use the same colour of thread as the fabric (ex : white tread on white fabric) but you can also use a tread which is a few darker or a few lighter than fabric but in the same familie. We do it a lot at present in France especially for vintage projects. I hope you will understand my "poor" english ! Bisous

I'm always learning new things here. Thanks 4 asking mops that ques. jrob *4U

I don't speak French, but I was guessing it was a version of tone on tone. Such a lovely, classic look (if you ask me).

I did not see your question until now, jrob, but you had a couple of good answers. It's same colour, often with different hues. In this case the hues were a bit extreme, so maybe I should not have used the expression. Flower for you all.

*4U

I don't speak French but it sounds like tone on tone. I do that often. A friend asked me to Emb on a few of her quilt blocks. I did a tone on tone Capital B in the center of the block and did family names on top in dk brown. The quilt is stunning. I'm still waiting for a picture.

All good answers. Now I've learned something new. A flower for you.