What is a split primary palette?
A split primary color palette is a slight derivation of the artist primary color palette, which is composed of red, yellow and blue. The split comes from taking those three colors and splitting them into warm and cool versions.

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As in the example above, the palette is often visualized with strong lines splitting the primary colors. The rest of the palette is developed by mixing the colors that are in the same divisional section. The &quot;formula&quot; comes out to be:

Warm Yellow + Warm Red -&gt; Orange Secondary

Cool Yellow + Cool Blue -&gt; Green Secondary

Warm Blue + Cool Red -&gt; Purple Secondary
If you try mixing across these lines, such as Warm Red + Warm Blue, the pigment interaction can cause the colors to become washed out or muddy. The same is true with your Secondaries. Don't mix your Purple with your Cool Blue. Sticking to the formula above and only mixing colors in the same 1/3rd sections keeps colors looking bright and intense.


Here's an example that shows how crossing the boundaries creates muddy colors (the center 3 colors):

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Aside from more vibrant colors, this type of palette allows you to more easily mix a full spectrum of colors. This is in part because the Cool Blue, Cool Red, and Cool Yellow are closer in value to Cyan, Magenta, and Yellow, the Subtractive Primaries (as seen in CMYK color standards). CMYK works better for mixing paint, because the paint gets its colors from pigment which &quot;absorbs&quot; light (and only reflects back the wavelength we perceive). The absorption is where the term subtractive comes into play.
From a physics standpoint, when you mix colors across your splits, you're mixing pigments that end up absorbing more wavelengths, instead of just more of the same wavelengths. For instance:

Warm Red reflects back Red, which is Magenta + Yellow, meaning it absorbs Cyan.
Warm Blue reflects back something closer to Cyan, which means it absorbs Magenta + Yellow.

So, if you combine those two together you have a color that absorbs Cyan and Magenta and Yellow, which is why it leaves you with a muddy brown or black (depending on your paint). Here's a diagram I made that shows this a little more clearly by comparing it directly to CMYK:

Notice how the Cool Yellow and Cool Red mix to make something closer to Red? And then mixing that with the Cool Blue leads to brown (pretty dark here, but compare it to the black center in the CMY set).
The additional benefit to using this type of palette is that it helps train you to naturally understand how mixing RBY/CMY works as you practice, while helping you avoid those mixes that lead to dim, muddied colors. As a plus, it could also save you some money by avoiding wasted paint!