Launch the ColorDrop app, pick an image from your library (or take a new picture), pinch to zoom in on an area, then touch and hold to bring up a popup showing you the RGB and hex versions of the pixel you are touching. Slide your finger around to visit other colors, all in real time. Press the Inspect button to reveal a boatload of detailed color space info on the current color.

If you work with color, this is definitely worth checking out. Runs on both iPhone and iPad.