I've seen European original art from this period where they turned the line art paper over and painted with watercolors on the back. I assume the colorist had the page on a light box so he could see where he was painting. The color art was separated and printed over the black. Most of the art I've seen colored this way was Italian. I'm not sure if French publishers used the same system. The final printing in this book looks a lot like the Italian books, though.
Link to the book:
Garry (serie 1) _ 132