Is it a warm or cool blue? Thank you, Debbie
SUSIE'S REPLY: Hi Debbie,
Most Phthalo Blue is PB15 and lean to the the greenish side of blue. In recent years some manufacturer (including M.Graham) have added an additional Phthalo's to their line that has been altered to have a more "purple" blue look. Look for the Phthalo Blue (RS) which stands for red shade.
From the manufacturers information I find that M.Graham's Phthalo Blue PB15:3 is altered to be more of a "true blue" and is a structural variant of Phthalo Blue PB15 (that produces more greenish tones. )
Here is a link to the M.Graham site with more technical information about their watercolors.
OK........that said, let's talk about warm vs cool.
Warmer or cooler is relative. It depends on the two colors you are comparing.
Red is warm and blue is cool... and its easy to see which is which.
All blues are cool when compared to any red. But when we compare several colors of the same hue (in this case blues) Manganese blue might be cooler than Antwerp blue and Ultramarine might be warmer than Cerulean blue. You have to visually compare them to see where they fit on the color wheel.
Because warm vs cool is relative to the two colors you are comparing, I find it's much easier to describe a color by naming the secondary hue it leans toward than to try to distinguish the difference by warmer or cooler. Example: New Gamboge is a orangish yellow and Hansa Yellow is a greenish yellow.
When comparing Phthalo blue(GS) to French Ultramarine Blue, Phthalo is slightly greener than Fr. Ultramarine blue which to my eye looks slightly purplish. So I say Phthalo is cooler, because on the color wheel it is closer to the coolest hue which is blue-blue-green.
The Fr Ultramarine is on the warmer side because it has a touch of red in it thus it farther away from the coolest color and closer to the warmer hues.
Now to answer your specific question: It is my opinion that M Graham's Phthalo 15.3 is very near a true blue. It has been altered so it doesn't lean toward a green hue as with the Phthalo Green Shade (GS) or a purple hue as the Phthalo Red Shade (RS). So I guess I'd say it is neither warm nor cool by itself. If we compared it to another blue I could tell you if it was warmer or cooler than the other blue hue.
I hope that is sufficient. It's not a simple answer.
Thanks for asking!