Chromodoris britoi from French Mediterranean

Thanks Marina,
I guess by 'dark spot' you mean the area where the pigment seems darker or more intense in the centre of the mantle. I think probably a more reliable external character would be the colour of the gills and rhinophores. In C. britoi the rhinophore clubs seem to be translucent white with a white line up the posterior midline, with a bluish purple line on either side and a third up the anterior midline. In some animals with a more intense blue pigmentation, the rhinophores appear blue but the posterior white line is still visible. The gills are also translucent with a bluish purple line up the inside and outside edge. In C. krohni both the gills and rhinophores seem to have a more uniform pigmentation, animals such as yours having a uniform translucent wine-red tinge wit some white speckling and in other animals illustrated on the Forum the colour becomes a more intense purplish red or even dark blue, but it is always a single colour.

I have included some close-ups of your photo of C. krohni in a separate message to show distinctive colour features of that species.
Best wishes,
Bill Rudman