"Phthalocyanine blue PB15 in its various shades (PB15:1 and PB15:6 are middle or reddish shades; PB15:3 is the greenish shade)...." so says Handprint, but we also have PB15:0 which is also a red shade. Confusing isn't it? Presumably there must be differences between the various numbers but apart from the green shade all the remainder are described as red shades. I've only got the one shown below but have rarely used it..
Left: Maimeri Primary Blue Cyan (PB15:3) full strength, middle heavily diluted, right Phalo Green (PG7) Michael Wilcox (Da Vinci). The diluted version tropical skies?
This is a lightfast, transparent, strongly staining, very dark valued, moderately intense blue pigment described as a `workhorse industrial blue colorant'. In watercolours virtually all manufacturers offer at least one, and sometimes more phalo paints, and it also features in student quality paints, where, with the addition of white, it is used to substitute for the dearer pigments in Cerulean and Cobalt Blue.
While many call it variations of phthalo blue it also features as Berlin Blue (Maimeri), Helio Blue (Schminke), Winsor Blue (W & N) and Primary Blue (Maimeri and Lukas). Once again don't be taken in by names - check the pigment numbers. All these shades are usually in series 1 so are the cheapest of the blue pigments. The colour itself is on the harsh side, especially if used in a strong mixture but is less `in your face' if heavily diluted. Watch the staining though on your palette as well as the painting.
Is this a worthwhile pigment? The fact that everyone offers it in one of the above numbers suggests it is and it is rated as a good mixer. Winsor & Newton, offer both Winsor Blue Red Shade (PB15:1) and Winsor Blue Green Shade (PB15:3). Lukas (now owned by Daler Rowney) have several versions and it also features in Turquoise shades, Maimeri, Rowney, Daniel Smith, where it is combined with phthalocyanine green (PG7 or PG36) .
If you decide to give it a try then proceed with caution as it is an unforgiving colour and thought needs to be given as to where and in what subject(s) it is suitable for. That's my view others may differ.