A light blue and a red that both seem clean on the palette might appear unexpectedly muddy after a few strokes. It’s not necessarily the case that acrylic paints don’t mix well. Color mud happens when too many pigments are blended together, your brush is carrying some of a previous mixture, or when you adjust a color too many times without a specific direction. It is much easier to mix cleaner colors if you have a sense of the color you want, whether warmer, cooler, lighter, darker, or less saturated?
It helps to work from a limited palette. Rather than having dozens of tubes available, select a few colors and understand how they interact with one another. Begin by mixing two colors and examine the result before adding any more. If the result feels too warm, introduce a small amount of the cooler color. It is not necessary to introduce a third color, as acrylic can be quite intense; a little goes a long way. Add color in small increments and mix well with a palette knife or brush.
Your tools can influence a color before you realize it. If your brush still has some green on it, it will dull an otherwise clean orange. Leftover dark color can also cause a new mid-tone to appear darker than intended. Rinse your brush when switching to unrelated color mixtures, remove excess moisture, and wipe the brush before applying a new color. Your mixing surface should also be clean. If multiple wet colors are crowding together on your palette, switch to a clean area instead of dragging paint through a dirty area of the palette.
Finally, be cautious with white and black. White can lighten a color, but can also shift it cooler or chalky. Black can darken very quickly and may also shift a color in an unexpected way. Check your current value against your reference photo before choosing to use white or black. Often, a lighter shade can be achieved with a small amount of white, or a darker mixture can be created with a deep hue from your limited palette. It is not about using no white or black, but understanding what white and black contribute beyond value.
An excellent exercise to help understand how colors behave is to create a simple color mixing chart using two colors. Start by placing your first color on one side of a line and your second color on the other. Blend several mixtures between them with varying ratios. Do not allow the mixtures to touch, and clean your brush or palette knife as needed. Once the swatches are dry, evaluate which colors have remained clear and which have turned muddy. Next, try the same exercise but add just a trace of a third color to the mixtures. This is a great way to pinpoint when a mixture becomes less intense.
If you notice a color becoming muddy, stop blending colors together to “fix” it. Create a fresh sample using fewer pigments. It can be quicker to start over with a small batch of color than to correct a mixture that has become overly complex. Take note of the last stage before a color turns muddy. This will give you more insight into managing color than all the tubes in the world.