Ultramarine' -from Latin ultra (beyond) and mare (sea)- used to refer to genuine Lapis Lazuli- a very expensive blue mineral pigment mined in Afghanistan. 'French Ultramarine' was used to refer to the synthetic version, which was developed for commercial use. However, over time synthetic ultramarine superseded Lapis Lazuli in availability and cost-effectiveness, and manufacturers gradually began to drop the 'french' from the name. However, both Daniel Smith and Winsor and Newton do both ultramarine blue and french ultramarine in their watercolour ranges! They are both PB29! What does it mean?! The difference may seem slight but it's important for watercolourists- the particles in french ultramarine are much bigger- meaning that it granulates more in watercolour than ultramarine blue. French ultramarine is slightly warmer (redder), whereas ultramarine blue is a little cooler (greener). To confuse things some more, Schmincke do an 'Ultramarine Finest' watercolour which, as the name suggests, is very finely ground to make a less transparent and less granulating ultramarine blue.
An explanation of the difference
Updated over a week ago