At first, halftone color codes might seem like a magic incantation or a bunch of nonsense, but when you break it down, it couldn't be simpler.
Every code in a color chart will have one or more letters and at times a number. If you're using ColorLab these codes will correspond to the brushes in the pack. Let's take a look at the Red codes so you can get an idea of how to read them.
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R2: The lightest or least concentrated in 20% halftone.
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R3: A mid-range concentration in 40% halftone.
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R4: The darkest or most concentrated in 60% halftone.
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R: Pure color or no halftone.
We can then apply that same logic to B or Blue, Y or Yellow, and K or Black.
So, the code R2Y2 that Matt uses to color the skin of the two people in his illustration is comprised of the Red 2 and Yellow 2 brushes.
If you're not using ColorLab you might find that their system works differently. In general, however, halftones will all use some variation of this technique.