Remember your elementary school art classes? You'd have paper and paints in front of you and a vision in your head, but the colors wouldn't always cooperate. Maybe you'd mix white and red to make pink, but an errant bit of green paint would get mixed in and would turn everything brown.
Yellow is an especially hard color with which to work and to mix, so trying to make yellow paint darker doesn't always work on the first attempt. Turning a bright, sunny yellow to a darker mustard shade without sacrificing that true yellow pigment requires a light touch.
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How Not to Darken Yellow Paint
Unless he's an artist with a solid understanding of color theory, if you ask someone to make one shade of paint darker, his first thought will probably be to add black. It's an understandable impulse, but it won't work.
Mixing black (or navy, gray or some other dark color) into yellow will only overpower the more delicate shade and create a muddy, grayish color. Dark colors strip the brightness from lighter colors. Just one drop of black paint could be enough to dampen any sunniness in a dollop of yellow paint.
Making Yellow Paint Darker
The goal when making yellow darker is to maintain the tone of the color so it's still recognizable as vibrant yellow. To do that with paints, the key is to add shades that complement yellow. Specifically, you can use orange, gold and purple, though red, green and brown can also be used to create darker shades of yellow. Which one you choose really depends on preference, so do some experimentation by mixing shades together and painting test swatches.
This method works not only with acrylic and oil paints but with oil pastels and watercolors too. Try first creating a layer of the light yellow followed by a few light brushes of an orange or purple hue. Use your finger to blend the colors.
Another option is to combine multiple shades of yellow together. For example, darken cadmium yellow light paint by mixing in some cadmium yellow medium paint. This is a surefire way to add some nuance and depth to your shade without diluting the tone at all.
Things to Keep in Mind
If you're new to working with paint, remember that paint may look darker once it's dry. Paint a small area with your yellow paint and let it dry completely before evaluating the shade.
When you're choosing colors to mix with yellow, think about whether you're trying to create more cool tones or warm tones. Focus on orange shades if you're trying to make warm yellow tones. Choose purples and greens to create cool tones.
Also, keep in mind that mixing new paint shades is fine when you're working on a small artistic project but isn't advisable when you're painting walls or other large areas. There's just too much risk that you'll end up with unevenly mixed paint or that you'll ruin the entire gallon by adding too much of one color. Save the "wrong" shade of yellow for another project and buy new paint in a shade that's just right.