If the color you want is visible somewhere on the screen, you can use the DigitalColor meter. It lives in /Applications/Utilities, and has built in commands to copy colors in any format. Choose the desired format from the color format popup, point to a pixel of the right color and hit shift-command-C. This copies the color as text so you can paste it anywhere.
If you have a picture of a color gradient open somewhere, you can use this method to pick colors from that.

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