There are two colors available at any time for use in drawing and painting: the foreground and the background colors. The color indicator is located in the toolbar window, just below the rows of tools. The upper left part of the indicator is filled by the foreground color, the bottom right one - with the background color.
By default the foreground is black and the background is white. To change a color, click on it and then proceed to pick a color via the color palette or the Eyedropper tool.
The button with two arrows to the right of the color indicator lets you swap the foreground and the background colors.