Instead of mixing paint on a palette and then applying it to a canvas, you can also build up color by painting glazes. It is a painting technique that requires a bit of patience as each glaze or layer needs to be totally dry before the next is applied, though with acrylics, of course, you don't have to wait very long.
You also need enough knowledge of color mixing or color theory to be able to predict what color(s) you're going to get.
This glazing demo shows how color on a leaf was built up from an initial layer of blue, into a leaf that's red and purple. Let's get started...
If you want to know more about the principles of painting glazes, read this FAQ on Painting Glazes using acrylics or oils.