Check the screen for existence of so-called "dead pixels". So called bad and defective pixels are defects of the electronic device which is perceiving or reproducing the image and having pixel structure. A more detailed explanation is in Wikipedia. Also see the ISO_13406-2 standard for more information.
Although APK downloads are available below to give you the choice, you should be aware that by installing that way you will not receive update notifications and it's a less secure way to download. We recommend that you install the F-Droid client and use that.Download F-Droid