I have pink pimples on my back and they turn to light black colour on my skin when they finish , please tell me what is this?

Hard to say without seeing them, and I don’t really want to see them, be honest, but are they maybe blackheads? Google it, maybe google “blackhead” on google images and see if the images match what’s going on with your skin. They’re very common.

Press down on the skin surrounding a spot. If you get a solid translucent yellowish column of stuff, you’re dealing with a blackhead.

This is a website dedicated to the appreciation of all of the various dermatological nightmares out there. It is in many respects a catalogue (though probably not a very complete one) with an oddly fanatical following. You may find your condition here.