Look down near the bottom of the toolbar and you should see an area with two overlapping squares of color. The square on top is called the foreground color. This is the color that you "paint" with when you draw using the brush or pencil tool, and it also has other purposes. The color partially hidden beneath the foreground color is the background color.
The default setting is black in the foreground and white in the background. You can return the colors to the default at any time by clicking on the tiny black and white icon to the lower left of the color squares. If you wish to reverse the order of the two colors, switching the foreground for the background, you can do this by clicking on the double-headed arrow to the upper right of the two squares.
There are several ways to change the foreground and background colors. The first way is to click directly on the squares. When you do so, the "Color Picker" dialogue box appears, as in the picture (right).
You can use the Color Picker dialogue box to set the color in several ways. If you hold your cursor over the large square filled with a gradient of color, it will turn into a circle. Click the gradient, and whatever color is in the center of the circle will be displayed in the upper half of the color icon to the right of the spectrum. The lower half of this color icon will remain set at the previous color so that you can compare the original with the new color you are selecting.
By moving the triangular arrows on the spectrum, you can change the color displayed in the gradient box. You can also click directly on the spectrum itself.
If you know the numerical value of the color you want, whether it is an RGB value, a CMYK value, a hexadecimal value (used when coding web pages), or some other value, you can type the number directly into the appropriate fields. You will probably use RGB values most frequently. An RGB value is made of three numbers, each ranging from 0 to 255. The first specifies the amount of red, the second green, and the third blue. If all three are set to 0, the result is black. If all three are set to 255, the result is white. If all three are the exact same number, the result is a shade of gray.
To the right of the color icon is a small cube icon with a tiny colored square beneath it. This is to indicate a "web safe" color. There is a limited selection of colors that are called "web safe"; if you are composing a web page, it is considered best to stick to this range of colors so that they will display properly for all users. The color in the tiny box below the cube will display the closest "web safe" color to the color you have chosen. Clicking on the tiny box will select that color.
Another color-related problem that crops up is the difference between the RGB spectrum and the CMYK spectrum. Working in RGB mode is fine if you intend your finished product to be displayed electronically. However, if you intend to print it out using a standard four-color printer, it will automatically be converted into CMYK mode first, so that the printer knows how much of each color ink to use. The difficulty here is that, with these four colors alone, the printer cannot reproduce the full range of colors that are available to you in RGB mode. In particular, it cannot reproduce the brightest colors, so the printed image will look more "dull" than it did on your monitor.
While you are working in RGB mode, if the color you select is not a color that can be printed accurately in CMYK mode, a warning exclamation point will appear above the "web safe" cube icon. Thus, if you intend to print the image, you should watch out for the warning icon and be aware that the color you see will not necessarily be the color you get.