True concolor remain uncommon, but are by far easier to find here than in the other areas.
As this form is defined by the conjunction of an absence of pattern with a single colour, it is thus an aberration compared to the normal form maculata, from which it derives so closely that naming this variant does not always seem of a great relevance.

Transition in yellow from maculata to concolor.

Like one could note with the shells coming from Le Croisic, there are bigger differences between maculata and lineata: in lineata, if any pattern disappears, a glazed dark line appears, which runs along the ribs. This line is a characteristic which nothing enabled to foresee in maculata.
One more thing: the line develops only on a plain colour background, usually white but not inevitably.

From left to right: lineata, maculata, and a quasi concolor from Le Croisic.
There is an obvious relationship between maculata and concolor, while the effort to deploy to pass from maculata to lineata makes necessary to have the transition under the eyes to be convinced that these two shells belong to the same species.