One of the main advantages of this color picker is that it can be scaled to any size preserving quality of the gradient of the picker and slide area. Moreover, the user is able to restyle the picker and slide area to fit their needs.
And what is the magic behind? Sorry, no magic, just normal (usually forgotten) SVG/VML. The size of the vector objects (picker and slide) is set to 100% width/height so that they are adjusted according to the outer divs. Registered callback is invoked whenever the color changes and it is passed a color in hexadecimal, hsv and rgb formats. That's it.
Try it yourself: Flexi Color Picker.