Just try a photo-editing app, I am sure they have this feature. If you don't know any you can use this one imagecolorizer.com. I used it, ones for work, so I am sure that you can take a screenshot with it. Just check it out. But if you don't mind my question why you want to do that? Because you can do a screenshot by using the special button that every keyboard have, so I don't see any point in using an app here. That's my opinion. Hope you will find my message useful. Have a nice one, my friend.