Blue poison dart frog
One of the things I noticed in the last workshop was how many people had Pthalo blue on their palettes and didn't know it. Pthalo is a powerful slightly greenish blue. I don't happen to like pthalo much, but you might. I feel it is out of step with the rest of my palette, being so much higher in pigmenting strength and that it can give an electric look that I don't particularly want in my somewhat reserved color. So I don't use it. But a lot of fine painters have and it is a permanant color. I am not opposed to modern pigments, I like quinacridone a lot, and there are a lot of great new red pigments.
BUT it is important to KNOW what is on your palette. If you are using pthalo, you need to know that. Many manufacturers either don't disclose on the tube that their paint is made with pthalo, or make it hard to figure out. Some companies force you to guess, divulging nothing about the contents of the tube. I want to use pure and named pigments. So I don't buy colors with names like astral blue or Hortensia. Pthalo is an inexpensive blue manufactured in enormous quantities. It is everywhere in the products and printed matter about us. It is also very cheap as pigments go.
Many of the colors that are counterfeited with pthalo are very expensive, like cerulean, cobalt or viridian. I give out a list of recomended colors before a workshop and suggest that they be the real thing and not a pthalo imitation. Probably half of the students show up with a thalo of one kind or another, and have no idea that is what they are using. So I asked each student if I could see their tube of viridian. Almost none of them were printed with the word hue (indicating a counterfeit) and on most of them the pigment code gave the content away. Pthalo is usually PB-7 or PB-15-3 or PB-36
Paint companies love the stuff and make a bewildering array of mixtures based on pthalo that you might not be aware are. There aren't that many blue pigments out there in the art trade and if the tube is filled with genuine viridian or cobalt, the manufacturer will be certain to boast of that on the label. Most of the proprietary blues and greens are pthalo, such as Winsor blue. If the paint is a blue or green and named something like astral blue, Sevres blue, or monastral blue, you can expect it to be a thalo based mixture. If you have any doubt or the tube doesn't say what it is, you can probably assume it is pthalo.
Sap green used to be made from buckthorn berries and I used it long ago. It was impermanent. Today, sap green is made of pthalo doctored with a little yellow. Permanent green is a pthalo color. Pthalo makes a lousy substitute for viridian, but viridian has become quite dear and as I said pthalo is cheap.
So check the small print on that tube of whelk blue and find out what you are really using.
I will be painting tomorrow at the Bethesda fountain in Central Park. If you are in the area, stop by and say hello. I expect to arrive at about 11;00 in the morning. You will recognize me because I am 6'4 and have shoulder length gray hair and I will be working on a Gloucester easel.