It sounds like you turned on a shader without meaning to. Assuming you’re using Retroarch, open the Quick Menu and scroll down to Shaders, then click that and see if Video Shaders is turned on. You can either turn it off, or you can choose a different shader for various effects. I use the gb-palette.glsl shader which makes games look green and dot matrix-y like the original system, but it’s only a matter of personal taste, go with whatever makes you happy.