Mix equal parts of cyan, magenta, and yellow acrylic paint
Add a small amount of black paint if the mix turns muddy instead of dark
Deepen the mixture with a touch more magenta or cyan as needed
Use ultramarine blue, burnt umber, and a little red to create a near-black
Adjust the mix with tiny amounts of white only if you need a softer dark tone
Test the color on scrap paper or canvas before using it in your artwork