How can I choose hexadecimal web colours (like #000000 and #fffff) to use in system and third party apps? (e.g. an image editor like GraphicConverter)
Skala Color is another solution, that works very well in the latest versions of Mac OSX.
You can select HEX to appear in place of the RGB code, which appears in the above picture.
I used Hex Color Picker for a long time but finally got tired of it trying to be smarter than me. Frequently I would type a single character and it filled in all 6 values for that same number; other times I would get through 2 characters and Hex Color Picker would repeat that. For whatever reason, I had to constantly copy/paste values into the picker, because it kept thinking it knew what I wanted (it didn't).
In newer versions of the stock OS X Colors utility: Select sliders from the toolbar and then "RGB Sliders" in the drop down - this will allow you to both read and enter RGB values hex:
Not sure in which version of OS X this was introduced, but it's relatively newish.