Latest over at ManiacDev is a collection of color pickers:
A somewhat different list than you can find at cocoacontrols.com.
Personally, we rather like the looks of one that didn’t make their list, IL Color Picker for iOS:
Source includes two controls that work in unison with each other to provide users the ability to pick color in a fashion identical to photoshop.
Designed not to use images or any other external dependencies beyond the classes…
… especially as “identical to photoshop” is a paradigm your users are likely to have a good chance of grasping. But choice is always good!
Scala Color: “An extraordinary color picker for designers and developers.”