Phthalo Blue is a super-powered blue. This powerful color usually comes out of the tube almost black, but it creates brilliant bright ceruleans when thinned to a glaze, and is also used to create gorgeous blue tints. Phthalo Blue comes in several sub-varieties, but the two main categories are red shade and green shade, which connote a subtle difference in the leaning of the blue. The varieties can also be identified using the number after the colon in the pigment code.

Phthalo Blue PB15:1 may be the most common variety of phthalo and is one of the "Red Shade" types (RS for short). This convention among blue pigments of a "red shade" and a "green shade" can be confusing. The red shade tends to lean more indigo and the green shade creates better cyans. In the case of Phthalo Blue, the Red Shade is often a lovely middle blue.

Most phthalos are very intensely tinting pigments that can overpower other colors, so a little goes a long way. A dot in a mixture can make a difference.

While generally it is not considered to be a concern for toxicity, we have read that it may be contaminated with very toxic impurities depending on the country of manufacture. These can include dioxins and PCBs. Older tubes should be avoided for this reason. For more, see Monona Rossol’s work on artist safety.