Mixing colors
Now you've got your primary colors, you're ready to start using them to make all the other colors you want to use in your painting, like violet and orange, puce and ecru, chartreuse and plum. You'll need to use your primary colors in various mixtures to create all these other colors.
To mix a color you need, first decide which two primary colors will most likely create the color you want. For example, if you need orange, choose red and yellow. Use your brush to lift some paint onto a blank area of the palette, then if you're anal wipe off your brush on a paper towel (I don't usually bother) and use the brush to grab some of the other color you want to use, and mix the two colors together with the same brush. Mix thoroughly if you want a uniform solid color, or mix less thoroughly if you want to see "streaks" of the different colors in your painting.
It will take a fair bit of practice to get good at mixing exactly the right color, but in the meantime you'll probably get some interesting "mistake" colors which can be instructional and fun to use too.
Be careful if mixing three or more colors together, you can easily create "mud" (a very dull brownish gray) this way.
To mix more vivid, intense colors, use the versions of the primary colors that are closer together on the color wheel. For example, to make a really vivid green, use the greener yellow (the cadmium lemon yellow rather than the cadmium yellow) and, if you have two different blues, use the greener blue rather than the more purple blue (I don't recommend using it, but phthalo blue is a greener blue than ultramarine). For a duller green, you'd use cadmium yellow and ultramarine blue, the colors farther away from green (a more orange yellow and a more purple blue).
To lighten colors, you can use titanium white. Be aware that in addition to lightening, it will also wash out the color's intensity, and it also tends to cool down the warmer colors (red, yellow, and orange). Try using a little yellow when lightening red colors to keep the intensity bright and to avoid getting too pink.
To get very pale colors, start with a big glob of white and add just a tiny speck of the other color to it. You'll soon find that certain colors, especially the reds, have a lot of "tinting strength", that is, a little bit goes a long way, especially when mixing it into white.
To darken colors, I don't like using black because it tends to dull colors a lot, so that's why the primary colors I recommend in my paint-buying article tend to be dark to start out with. You can make a very nice almost-black color by mixing ultramarine blue with alizarin crimson.
You can dull down ("neutralize") a color that's too intense by using its "complement" (the color opposite it on the color wheel. For example, use a little dab of blue to tone down a too-intense orange.
You can mix a huge range of neutral colors like beiges and creams and browns by carefully mixing all three primaries (red, yellow, and blue) with white in varying proportions. This can be very challenging and takes a lot of experimentation, so be patient with this part.
You'll need to experiment to see what different colors you can mix. However, there are certain colors that can be very hard to mix from primaries. If there's a color you need that you just can't seem to get by mixing, then it's probably worth it to go back to the art store and buy a tube of it premixed. But it was worth it to figure this out for yourself, wasn't it? Otherwise, imagine how many unnecessary tubes of paint you might have bought!
Articles on Acrylic Painting - Getting Started
Why paint with acrylics?
Basic equipment for acrylic painting
Buying acrylic paints
Painting with acrylics
Mixing colors