Folks may remember the last painting I posted - the one called Rocky Beach at Fossil Cove
with the sun glinting on the water and all the wet pebbles. I really liked the blue sea colour in that. I had an old, almost empty tube of W&N Prussian blue that I cannot remember ever using. But something told me it might be the right blue for the sea in that painting and I think it was.
I've started a new painting that has sea of a similar blue (but not all the glare and sparkles) and Prussian again looked right. Alas, none left! So I've experimented with mixing something approximating Prussian using Ultramarine, some Burnt Umber and a little Phthalo green and it's very close to Prussian which has a blackish mass tone and a greenish undertone. When mixed with Titanium white you get some dreamy cool shades of blue. (For an idea of what Prussian looks like see below which is Prussian from the Langridge colour chart) My mix looks very similar. If you look carefully at the pic below you can see a richer blue between the two swatches where they mix. That is the colour I was after. Phthalo blue was too chromatically strident, and Ultramarine too warm, but Prussian was just right. I'll post a photo of my mix so you can compare.
I'm not sure I could have mixed this using only Mark's 5 colour limited pallet. There may be other ways to mix this but if anyone wants to try Prussian but doesn't have a tube handy, my mixture of Ultramarine, some Burnt Umber and a little Phthalo green is not a bad approximation.