We already know how to choose a color using the Swatches palette. Unfortunately, the range of choice in it is very narrow. So, what about the other colors? It may be a very simple task if you can find the required color on your picture.
§ 1. From the Toolbox, select the Eyedropper. By default, this tool reads colors of individual pixels. In some cases, though, it is better to choose a larger sample size (e.g., to minimize influence of accidental colors).
§ 2. Find a color on the image that you'd like to paint with and click it with the Eyedropper. Now look at the foreground color box (the upper square at the bottom of the Toolbox) - its color has been changed. Now we may activate the Brush Tool and paint with the selected color.
§ 3. While working with the Brush or some other painting tool, you may temporarily activate the Eyedropper by holding down [Alt] key. Once the required color is selected, release the keyboard key and continue painting.
§ 4. Very often, however, we are not completely satisfied with the available colors. Click on the foreground box to call the Color Picker.
§ 5. Click within the color field to define the most suitable color. If you can't see the required color in the color field, select "H", "S", "B", "R", or some other radio button to alter the color presentation model. Then, if necessary, fine-tune the color hue (H), saturation (S), amount of red constituent (R), etc. by moving the vertical slider.
There are quite a number of ways to define a color with this tool; the movie shows only one example.