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When drawing with one of the draw primative tools, you can use the right mouse button to draw with the left color, and the right mouse button to draw with the right color. Or, you can use a gradient. The line colors are the colors used with the freehand tool and the spraypaint tool, and the colors that outline filled shapes like ellipses and rectangles. The fill colors are used only to fill shapes. If you don't want to outline a shape, choose Transparent for the line color; if you don't want to fill a shape, choose Transparent for the Fill color.