There is no way to change the selection color within AppStore limitation. In fact, considering MobileNotes.app's selection background color is also light blue despite the caret is brown, we can see that Apple doesn't intend to allow non-blue selection background color.
However, you can calculate the rectangles occupied by the selection, then make a colored transparent non-interacting UIView on top of it to simulate a change of background color.
You can change the selection color globally (within your app) by overriding the undocumented method +[UIColor selectionTintColor]
(e.g. using method_setImplementation
, etc.).