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I suspect that this is absent because Paintbrush is utilizing OS X's color picker, and Apple did not see fit to include one -- but since the "web safe" days are, for the most part, long over, it would be very nice to have. I wonder if there's a dashboard widget to convert hexadecimal to RGB or HSB?
That is correct: OS X's color picker does not include any useful conversion for hex. However, you can easily interchange RGB, HSB, and CMYK values with the picker: simply choose the second titlebar option in the picker itself and enter a value into either the RGB, HSB, or CMYK sliders. When you switch between the three, the new values will represent the same color.
If you're looking for a dedicated solution, I recommend ColourMod's free Dashboard widget, which can be downloaded here: http://colourmod.com/dashboard/. It's a very powerful tool, with support for Hex, RGB, CMYK, and HSB values.
Lead Developer, Paintbrush