At least 5 years ago I stumbled on to a great art tool for the iPhone and iPad called "Value Viewer". It allowed you to simplify images by turning them to grayscale, two-tone, and three tome images. After all, good art is about values, color is just the icing.
The developer decided not to continue support for future ios versions and I was without this tool until I discovered "See Value". It works a lot like Value Viewer and also has a crosshair tool in grayscale mode that lets you hover over a spot to see its relative value. All images can be saved to the camera roll if you pay for the 99 cent upgrade.
Here's a screenshot of the "See Value" in action:
Definitely worth 99 cents and really useful for both studio and Plein air painters!