OS X: If you see a colour on the web or in an app that you particularly like and want to save for later, Sip makes it incredibly fast and easy to store it for later use.
Sip lives in your menu bar, but you can invoke it at any time with a customisable keyboard shortcut. Activate the app, and you'll get a magnifying glass to zoom in on any pixel on you screen. Click on the one you like, and its colour code will copy to your clipboard to use in the image editor of your choice. You can choose your preferred colour code format (HTML hex values, CMYK, RGB, etc.) in the app's settings, and switch between your favourites quickly in the menu bar dropdown. If you're looking for something similar on Windows, Instant Eyedropper is an oldie but goodie.