Blue was in short supply and used sparingly in art throughout the 18-1900s. As times have changed and color science evolves, we exist in a time overwhelmed with a wide variety of blues: ultramarine, pthalo, cerulean, cyan, indigo, cobalt, navy, azure, cornflower, and so many more!
While this is something artists should celebrate as we have more options today for creative expression than ever before, it comes at a cost. Artists face decision fatigue whenever they visit the paint store or their palette.
This brings us to some pretty big questions: which blue should you choose? is there a right and wrong shade of blue? and how can we apply it to our art?
Which Blue to Choose:
To mix a wide variety of colors using the traditional primaries, you need a warm and cool version of each. For red and yellow these are easy to identify. Alizarin crimson and rose lake madder lean toward purple which makes them cool, while cadmium red and vermillion hues lean closer to orange which makes them warm. For yellow, an electric lemon yellow pushes toward cool green, while a dandelion or school bus yellow leans toward a warm orange. But for blue? Cue one of the biggest debates in color theory history.
Artists argue whether ultramarine blue or pthalo blue is warm or cool. Ultramarine blue leans toward indigo and deep violet and pthalo leans toward teal and green. If we continue further on the color wheel, ultramarine is closer to red than pthalo blue so clearly, that's the warmer blue, right? Well if we continue farther along the color wheel in the opposite direction we find that pthalo blue is closer to yellow than ultramarine. Which is also a warm color.
After extensive research, I've come to the conclusion: functionally, ultramarine and pthalo blue are both warm blues. Out of the three primaries, blue is the only cool hue. Either direction you take it will push it toward a warmer tone. The only actual cool blue would be a true primary or cobalt blue.
Is There A Right & Wrong Shade of Blue:
Which of the warm blues should go with a cool primary blue? Should we use it in addition to both of our warmer blues? Does this mean we should only use a true primary blue? Can we mix a primary blue from ultramarine and pthalo? Short answer to all of these questions? Yes. Ultimately there's no wrong way to make art. You can choose a blue based on intuition alone and a lot of us do with great success, but that's not why you're here reading about color theory is it? And I don't want to leave you with more questions than you came in with.
We know we need a warm and cool version of each primary in your palette to mix the widest variety of colors. The key here is variety. Regardless of whether you find a shade of blue warm or cool, having a blue that leans toward purple and one that leans toward green is important for that variety whether it comes right out of the tube or you mix the shades yourself.
Applying this Knowledge:
To know which shade of blue to use in our art, we need to take a look at color harmonies and context - the relationship blue has to other colors on the color wheel and within your art.
Using the popular complementary color palette, when paired with a red-orange, a blue-green would be the right blue for ultimate color harmony and paired with a yellow-green, a red-violet would be the choice.
In a tetradic or double complementary palette, all four of those colors could work in harmony as seen below in the background of this painting.
There are a few instances where both shades of blue are used in harmony. A split complementary palette with orange uses both a blue-green and a blue-violet, and an analogous color palette can use all shades of blue with blue-violet, true blue, and blue-green. Using that analogous palette in moderation, one could argue that it's actually monochromatic with only slight variations in hue and relying heavily on saturation and value to carry the emphasis of the artwork.
Now that we've discussed the cases where blues can work, when does a certain shade of blue not work? While I don't believe there's a wrong way to do art, there are reasons why a shade of blue may complicate your artistic process.
Saturation & Value
Even more important than color, value and saturation must be considered. Not all saturated colors are created equal in regard to value. Most visibly, a saturated yellow is quite light in value compared to a saturated violet. Looking at the color wheel, we can see that they sit as opposite complements. Armed with this knowledge we can discover that red and blue (both neighbors to violet) will saturate to a darker value, while orange and green, (neighbors to yellow) will saturate to a lighter value. Applied to blue, a saturated ultramarine blue with a bias to violet will saturate to a darker value than a pthalo blue with a bias to green.
Highlights & Shadows
Armed with this information, artists may choose warm and cool tones to create a push & pull within their work. There is no steadfast rule that all highlights should be warm & all shadows should be cool, but highlights and shadows should be opposing on the color wheel. Selecting a warm or cool tone for your highlights and shadows depends heavily on your light source.
A sunny outdoor scene may have warm sun rays and cool shadows. On the other hand, artificial fluorescent bulbs may produce a cool light creating cool highlights and warmer shadows. This rule can be heavily exaggerated with saturated colored light sources, for example, a magenta light would produce a green shadow.