So far in the book you have changed the color of a shape, lines and text by first clicking on a color well, and then selecting the preferred color. This process can be quite tedious, and OmniGraffle will, for most tasks involving color-change, let you drag a color from the color picker directly onto a shape, the text or a line.
Start by drawing a rectangular shape on your canvas and place the text This text belongs to this shape inside. The stroke thickness is set to 5 points, and the font is 18 points Helvetica.
You are now ready for some color picker magic.
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