After I wrote the post comparing Ultramarine to Cobalt, I've had several people ask, "What about Phthalo blue?"
Great question! Phthalo blue (aka Thalo blue) is an abbreviated name for paints made with the pigment Phthalocyanine Blue, or PB15. This blue is a longstanding favorite of many watercolor painters and seems to be an extremely popular choice for a watercolor palette.
Each watercolor manufacturer often produces multiple Phthalo blue colors, and these are commonly divided by whether they have a green or a red undertone. Often you'll find them labeled Phthalo Blue Red Shade (RS) or Phthalo Blue Green Shade (GS). The pigment numbers* can also help you distinguish between the two. PB15:1 and PB15:6 are middle blue or reddish shades; PB15:3 is the greenish shade.
I wanted to try Phthalo RS because it is is often touted as a good, non-granulating option for Ultramarine, but I also wanted to give Phthalo GS a go.
Bruce at Handprint gives good info on the various properties of the major brands of Phthalo blue watercolors, but I already had Daniel Smith RS (PB 15) and Sennelier (which doesn't specify but it's PB15:3) on hand.
So let's compare Phthalo blue red shade to Phthalo blue green shade, see how they perform, and look at some common mixes.
*If you're a bit confused by what these letters and numbers mean, this post explains things.
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Phthalo Blue's Performance
Until now, I have avoided Phathlo blue because it is a strong pigment that is highly staining, and that attribute can be a bit difficult in the hands of a sloppy painter in the field. Also, I assumed a staining blue would be difficult to lift when creating clouds, etc.
However, my reasoning is extremely faulty because my palette is peppered with staining colors including Phthalo blue's BFF, Phthalo green. And just check out how beautifully Phthalo lifted out to make clouds!
In the top photo, I applied Phthalo blue RS to wet paper and lifted out the cloud pattern with a damp rag. In the bottom photo, I applied Phthalo blue GS to dry paper, yet it also lifted out easily with a damp rag.
I did NOT let either pigment dry before lifting (work quickly!) and I used a high quality watercolor paper. I think these two things are the keys to achieving such great results when lifting, especially when using staining colors.
Alrighty, so I was wrong. It was time to give Phthalo blue a try in mixes and compare it to some other blues.
Comparing Blues: Phthalo vs Cobalt vs Ultramarine
You've probably already noticed the different colors of Phthalo in the cloud photos above, but when exploring a watercolor, I always try to paint out a large swatch so I can see how the color reacts across a page, so let's take a look. And just for comparison, I'll also swatch out ultramarine and cobalt.
When these blue watercolors are side by side, it's easy to see how greatly they vary and (depending on your computer monitor) which shades may recede or advance in a painting. Instead of focusing on "cool" or "warm" descriptions which are often based on opinion, see if you can tell which blues enjoy the limelight versus which blues seem content singing background vocals.
The lead photo of this older post is a perfect example of a fatal blue error. We had been traveling to the Blue Ridge mountains to look for land, and I tried to capture these high country hills using my beloved ultramarine.
Back when I painted this scene, I knew something wasn't right. Now that I know better, I can see how wrong limelight-loving ultramarine was for these soft, unpretentious mountains. (The mountain on the far right looks like its marching in to take over!)
If you continue to scroll down through the various sky paintings, its easy to see how various blues tend to appear heavier and advance on the page or appear lighter and fade into the background. As you begin to see this, you can plan your blues in your paintings accordingly.
Watercolor Mixes: Phathalo Blues & Ultramarine
Of course, mixing a blue with another color can alter it drastically, so let's look at how both Phthalo blues mix out, and I'll also include ultramarine so you can compare it to Phthalo blue RS.
To give you the best idea of the range possible for each mix, I displayed two colors per mix: the first (left) is the mixed color dominant, the second mix (right) is the blue dominant.
I've heard through the watercolor grapevine that Phthalo blue GS is a better mixer than RS, but based on my experiences, I disagree. No matter which blue one chooses, a beautiful range is easily possible, and results aren't so varied that choosing one blue over the other should make or break a painting. All of these blues could be further modified by additional mixing.
More important than any particular blue is a painter's personal preference and knowledge of the subject. For example, are cool/receding hues desired? Go with Phthalo blue RS or cobalt. If you love granulation or want a blue that will pop, choose ultramarine. If you constantly need a spring green or clear teal, hang out with Phthalo blue GS.
I'm not sure a palette (or a budget!) needs all of these, so go with what you love and then mix what you need. Your palette and your paintings will thank you.