This is the best visual magnifier app that I have found. It is published by a reputable company, and has some nice features.
You can magnify an image up to 16 times without losing much definition, and can use a variety of coloured overlays. You cannot choose the exact shade of your overlay, which is a shame, but nevertheless it may be of use to some dyslexic learners as well as visually impaired learners, who are the main target audience.
This short video gives you a quick tour of the app:
At the time of writing, this app was free to download. For more information, visit the publisher’s website.