An Alternative to Phthalo Blue?

History of the Colors Phthalo Blue and Ultramarine, Real and Synthetic

Limited pallete painting project
 Edgar Coudal

It's a color conundrum: Can you use a different blue for a limited-palette project if phthalo blue isn't a color you already have? Can ultramarine, cobalt, or cerulean blue substitute well for it? It would be churlish to say no; if you don't have phthalo blue, you may substitute ultramarine.
Ultramarine is the best alternative because that color is also a transparent pigment with a good tinting strength. Cobalt is transparent but has a weak tinting strength, and cerulean blue is only semitransparent, also with a weak tinting strength. The disadvantage of ultramarine blue over phthalo blue, though, is that it doesn't make as deep of a dark shade on its own.
But first check that you don't have phthalo blue lurking around under one of its other names, such as thalo blue, monestial blue, Winsor blue, monastral blue, phthalocyanine blue, intense blue, Old Holland blue, or Rembrandt blue. Check the label to see if the tube contains PB 15, and then you've got phthalo blue.

What the Heck Does 'Phthalo' Mean, Anyway?

The name of the color comes from its chemical composition, from its class of insoluble pigments called phthalocyanines. The blue was synthesized by Imperial Chemical Industries, introduced to the wider public in a 1935 article in the journal Nature, which championed its ability to make "much brighter greens and purples": 

"Monastral Fast Blue BS has none of the various drawbacks of the long-known Prussian blue and ultramarine or the more recently discovered blue lakes derived from coal tar colours, and will inevitably replace them in paints, distempers, varnishes, enamels, in textile printing and in the pigmentation of rubber, plastics and cements."

Chemically, it's composed of rings of nitrogen and carbon atoms around a copper atom. 

What's Ultramarine, Then?

Ultramarine pigment was first created by grinding up the semiprecious stone lapiz lazuli, found in Afghanistan and Chile. Used in Afghanistan since the 6th century, its most widespread European use happened in the late Middle Ages of the 14th and 15th centuries. Italian panel paintings and illuminated manuscripts featured the pigment, which was imported there via Venice. Its use required the deep pockets of the church; European artists there couldn't afford it, as its rarity demanded a premium to say the least. As late as the late 1820s or 1830s in Paris, it cost between 3,000 to 5,000 francs per pound.

In 1787 Johann Wolfgang von Goethe knew of an ultramarine substitute that was created by scraping a blue residue off lime kiln walls near Palermo, Italy. Because the real ultramarine blue pigment was so expensive, pursuit of an artificial substitute was well documented, and a prize was offered to chemists who could come up with a compound resembling the chemical composition of the real thing. Artificial ultramarine pigment was ultimately first synthetically produced in the 1820s in Europe from china clay, sodium carbonate, and sulfur, plus some silica and rosin.