Pixel value grabbing advice please

Hi there,

OpenGL can be a bit restricting, which also makes it difficult to research good working practices. I know that this question gets asked often on different threads, but i want to ensure I’m exploring in the right direction for iOS.
I am working on a project where we want to display an image which will have been manipulated, then use touch positions to return pixel values. This will be a small part of the project, so need to find a very efficient approach, it doesn’t need to be exact, or even be pixel perfect (i.e. the sampled image could be much lower res than what is displayed)

Thanks in advance!