Often mislabeled as lightfast in watercolors. Unfortunately PB27 always fades in sun, but recovers in shade. Manufacturers who are aware of the issue may mark it as stable (LF I /BW8/max star lightfastness depending on the country) due to it's ability to regain color after
being removed from light. Paint should not need to be taken off the wall for a nap in a dark corner for it to be considered lightfast. I avoid this pigment if planning to sell a painting or display it in a gallery, as a buyer is likely to hang it in a well-lit room. It is only stable in shady museum lighting conditions. Artwork made with Prussian Blue WILL fade from window lighting, which can be noticeable within weeks in rooms with large windows or locations closer to the equator.
"BUT Kim, x brand of Prussian Blue is lightfast!" I'm sorry for the bad news, but it's chemically impossible for PB27 to be totally stable. It is produced by oxidation of ferrous ferrocyanide salts. Sun bleaches the FeIII (Iron) which requires time to re-oxidize away from light. Prussian has a reaction to UV light on a chemical level, which is only reversible over time in darkness. Don't take my word for it though, feel free to do your own testing, google search or do some PB27 research on Dick Blick, Handprint or other reputable sources that will confirm this. I realize that most artists and paint companies trust ASTM lightfast ratings, but I advise caution because their testing methods are limited. They do not always correctly distinguish between masstone and diluted, even when a pigment has been proven to fade in lower % tints. They also test mixes with white paint, which is not the same as just diluting with water as in watercolor painting. It's also important to note that sometimes a larger company who supplies pigment powder to the paint manufacturer supplies them with a lightfast rating (such as ASTM I-IV, Blue Wool 1-8 Scale or a 3-5 star system) which the paint maker may not double check for accuracy.
Each Prussian blue watercolor I've tested in every professional artist grade brand (including Daniel Smith, Mission Gold, Paul Rubens, W&N along with student grade brands) have all faded with UV exposure within 1-3 months. This is generally a fault with the ingredient, PB27. Fun thing to see though, is if you put it in a shady drawer for a week the color will almost regain full strength. The only variance between brands is how MUCH it will recover AFTER fading. Prussian Blue takes time to become darker again in the shade. Some brands can make a nearly-full recovery in several weeks, which may be due to the differences in pigment source and any protection from binder additives. Overall, PB27 is a very unique and interesting pigment, but one that is not reliably stable. It is likely that the original rating came from some one who collected results and let them sit in the shade before comparing them, or decided that since it was able to recover it was OK. To me this simply defies the definition of a "lightfast" rating. If the color is not stable in light, I consider it fugitive.
PB29 Ultramarine Blue: Also known as French Ultramarine, Lapis Lazuli (genuine or hue), Cobalt Blue (hue) or Permanent Blue. Non-hazardous (low to no toxicity in humans). Typically lightfast (LFI / BW8) but care must be taken to use acid-free paper, as sulfur-based pigments will discolor when exposed to acids. Ultramarine Blue is a replacement for the chemically identical Lapis Lazuli (a semi-precious stone). Daniel Smith still offers Lapis Lazuli genuine, the naturally occurring version of this pigment used in many famous paintings throughout history. It used to be reserved for the very wealthy, until chemist Jean Baptiste Guimet came up with a formula to make it synthetically in France during the 1820s. This took a surprising number of ingredients including sulfur, kaolin clay, silica, charcoal, sodium carbonate and sulfate. The ratio and heat used to create the material, and the exposure to oxygen, results in a variety of deep blue colors.
Variants: Ultramarine comes in multiple shades such as "deep" and "GS" (green shade) and when further exposed to heat and ammonium chloride (Salammoniac) it turns slightly violet. It is then renamed Ultramarine Violet PV15. Winsor and Newton have made a color called "Smalt" without the word hue. Smalt is incredibly misleading to those seeking historical pigments, as it is actually made from Ultramarine Violet PV15 and NOT genuine Smalt PB32. When you add hydrochloric acid, Ultramarine turns into a delicate pale pink called Ultramarine Pink PR259. Be aware that all Ultramarines are generally regarded as lightfast. However, "Ultramarine Green" PG55 which may be found as a raw material pigment powder, handmade watercolor or from the company "Kremer Pigment" is not lightfast. My tests have shown this PG55 version to be surprisingly fugitive.
Mixes: An incredibly useful color for mixing and often included in both beginner and professional watercolor sets. Ultramarine Blue can be mixed with PBr7 to create an ideal neutral gray. Another popular mix is with PV19, where it makes deep royal purples. Daniel Smith's mixes have reliable texture effects in wet washes, as the PB29 granulates and separates from the PV19.