Mix Your Own: Prussian Blue

Prussian Blue (PB27) is a pretty, mid-chroma deep blue that can attain dark values and mixes fabulous greens. One of the oldest synthetic colors, it’s found in many classic 18th/19th/20th century palettes. Unfortunately, it has a tendency to fade in UV light, though it can recharge itself in darkness. If we want to avoid Prussian Blue for lightfastness reasons, or just to save space on the palette, what mixes can we use?

Jane Blundell wrote in Watercolor Comparison: Blues that Prussian Blue is “easily mixed with Phthalo Blue and a warm red.” But is it?

Well I don’t find it that easy, but maybe my standards are too high! I struggled to mix a slightly muted blue that was neither too green nor too violet for the Prussian Blue hue.

Attempt 1: Phthalos + Warm Reds

Next to the Prussian Blue sample at the top, you can see Phthalo Blue Green Shade and Phthalo Blue Red Shade, which I used in my mixes. All of the mixes have far more blue than the other mixer so that it stays blue instead of becoming gray or violet.

Left Column: Phthalo Blue Green Shade

• PBGS + Pyrrol Scarlet (PR255): A nice dark/muted turquoise, but greener than Prussian Blue.
• PBGS + Perylene Red (PR178): Similar, too green.
• PBGS + Pyrrol Red (PR254): Similar, too green.

Right Column: Phthalo Blue Red Shade

• PBRS + Pyrrol Scarlet (PR255): Reasonable, but more muted and grayish, without enough green undertone (which Prussian Blue does have, just not as much as PBGS).
• PBRS + Perylene Red (PR178): Similar, too red.
• PBRS + Pyrrol Red (PR254): Similar, a muted blue, but no green undertone.

Attempt 2: Phthalos + Grab Bag

Left Column: Phthalo Blue Green Shade

• PBGS + Payne’s Gray: Straightforwardly desaturating and darkening with PBGS with gray also leads to be too green, but I don’t dislike it.
• PBGS + Quin Violet: Quin Violet moves the PBGS more to a mid-chroma middle blue, not unlike unmixed PBRS.
• PBGS + Venetian Red: I was shocked at how teal this mix was; it seems to become even greener than plain PBGS and to settle into a deep, dark peacock blue. I like it a lot but it’s not Prussian Blue.

Right Column: Phthalo Blue Red Shade

• PBRS + Payne’s Gray: Payne’s Gray leaves PBRS’ slight green undertone intact more than the red mixers do, which makes this one of the closest matches to Prussian Blue. It is still less greenish, slightly.
• PBRS + Quinacridone Burnt Orange: Rather than mix with Quin Violet (which already made the PBGS not green enough), I decided to mix with an orangey mixer. I was shocked at how much green QBO brought to this mix, making it much too green. It is lovely, though.
• PBRS + Venetian Red: Probably one of the closest matches in masstone, though the dilute is too gray and desaturated.

Attempt 3: Perylene Maroon + Dark Blues

On this page I added Indanthrone Blue and Indigo as comparison colors to show was a warmer and greener/darker blue look like. However I continued my convention of using Phthalos as the base, with Green Shade on the left and Red Shade on the right.

Left Column: Phthalo Blue Green Shade

• PBGS + Perylene Maroon: Perylene Maroon definitely seems to add yellowness to the mix, which is odd! This is one of the more greenish/turquoise mixes yet. I like it, but it’s not Prussian Blue.
• PBGS + Indanthrone Blue: I went the opposite direction and tried to add a color that would pull PBGS in a less green direction. Indanthrone is a dark blue, like Prussian Blue, but it is too violet-toned, so adding it to PBGS seemed like a reasonable idea. I find the result not dark enough and too high-chroma, though it’s a nice strong blue.

Right Column: Phthalo Blue Red Shade

• PBRS + Perylene Maroon: You would hope that if PBGS+PM was too green, PBRS+PM would be just right. Unfortunately, I find it too grayish/desaturated. It is closer to Indigo than to Prussian Blue.
• PBRS + Indigo: Similar reasoning to adding Indanthrone to PBGS: I added a dark blue that is too green/low-chroma, hoping to balance PBRS’ violet undertone and high chroma. This is the closest match to Prussian Blue.

Conclusion

I found a lot of nice dark blues, though it was not easy to make a precise match to Prussian Blue.

In general, Phthalo Blue Green Shade mixes were too green, and Red Shade mixes not green enough. This was especially true in the mixes with warm reds. The mixed PBGS colors tended to look more like Phthalo Turquoise, while the PBRS mixes look more like denim or Indanthrone Blue.

The closest matches came from Phthalo Blue Red Shade + Indigo, and to a lesser extend Phthalo Blue Red Shade + Payne’s Gray. It’s not surprising that those mixes are similar. The Holbein Indigo & Payne’s Gray I am using have the same component mix: actually they are a mix of Phthalo Blue Green Shade + black + a little violet. Indigo has more of the PBGS. Since I consistently found that the hue of Prussian Blue was in between PBGS and PBRS in greenness, it makes sense that mixing them together helps. Adding black lowers the chroma and increases the darkness.

As always, color-matching a particular color is somewhat arbitrary goal; if you are simply looking for a nice deep blue, you might find plenty mixes here that meet your needs. I found myself particularly drawn to the deep, rich turquoise/peacock blue colors from Phthalo Blue Green Shade + scarlet (notably Perylene Maroon, Pyrrol Scarlet, or Venetian Red). Mixes like this would also probably retain Prussian Blue’s superpower of mixing great greens.