The three primaries are unique colors cannot be mixed, but a few others as well.
Many artists are told that they only need the three primary colors, blue, red, and yellow to paint. In theory, we can create all the colors by mixing these three primaries, even black. In reality, however, some colors are not possible to mix with the primaries available on the market. Let’s look at these unique colors in order. Primary (red & pink, blue, yellow), Secondary (Orange, Green, Purple), Tertiary (red-orange, yellow-orange, yellow-green, blue-green, blue-violet, and red-violet) and the black monster!
Having a bright pink is really hard to do with any red colors. This happens even if the red isn’t red-violet like a Crimson or red-orange like a Cadmium. The most middle red that I found is the Bright Red of Winsor & Newton:
But, as soon as you put some white in the W&N Bright Red (or any other reds), you get a pink but never a high-intensity bright pink.
To get this unique color, you need something that is already bright pink out of the tube like Permanent Rose (made with quinacridone):
A few other colors are nearly impossible to mix from the three primaries. Phthalo green (made with Phthalocyanine) is one of them:
PURPLE (OR VIOLET)
So is cobalt violet (made from cobalt phosphate, or cobalt ammonium phosphate)
Even if purple (violet) and green are secondary colors and you should technically be able to mix true secondaries using true primaries. In reality, these secondary colors made from primary colors don’t get as intense as the phthalo green or cobalt violet above.
This is mostly because we don’t have absolutely true primaries in tubes. Take Ultramarine Blue of Cobalt Blue, for example, they both have a slight shift toward violet. So, even if you find an absolutely true primary yellow, the result will not be absolutely true green because of that shift in the blue.
The third secondary color, orange, is the one that is the easiest to make using a cadmium red that naturally tends toward the orange spectrum. In fact, all cadmium (yellow, orange, red) are made from the same family of pigments, (sulfides and sulfoselenides). This is why they mix so well together without losing color intensity.
TERTIARY COLORS (mix of 1 primary and 1 secondary)
Under normal circumstances, these can be made from the primary colors, if you have high-quality pigments that are close enough to true primaries
Black is the easiest and the hardest color to make. OK, I know some people are cringing, black is not a “color” or black should not be used in painting. Black can be all the colors mixed together in the right proportions or the absence of color all at the same time. Most Impressionists were against the use of black. This idea continues to influence many artists even today. In fact, since we can make a very decent black using a number of combinations (see my post about mixing black), there is no reason why the black from a tube would behave any differently when mixed with colors.
To be practical, Cadmium red and Viridian make a good black. Burnt Sienna and Ultramarine Blue work well together to make another good black. The advantage of working with complementary colors to make black is that you can shift your black toward a warmer or cooler tone by dosing the two complementary colors.
The hard part is that you never get an absolutely 100% dark black, for the same reason as stated above. To get the darkest black you need to get it out of a tube.
Are there other colors that cannot be mixed? YES…
Some colors are so transparent that the EFFECT they have cannot be achieved using a mix of colors that have opacity in them.
I use a type of Gold color that can never be mixed, why? Because it has other non-color elements in it such as reflective particles.
Apart from these non-color elements and transparency qualities, we should be able to mix pretty much every other color, especially, grays, browns, earthy colors, etc.