I haven't had time to look, but I've never seen one appear on the iPhone so for myself I'm going to assume it's not there. In fact, quite a lot of what we know as Cocoa seems to have been left out, or at least not wrapped in Objective-C (yet?). However, it shouldn't be too hard to make your own color panel. All you technically need is a view with 1-3 text fields and perhaps a color swatch drawn to confirm the color. Then you could fancy it up as you go along. Sliding with your finger to change values would be cool, as would presenting a color gamut and dragging your finger in two dimensions to set hue and saturation.