I was on my Minecraft menu and I went onto one of my worlds, and everything was shimmering and changing color!
Why is my Minecraft world changing color?
Katoptris
A few version updates ago Mojang added different textures built in to Minecraft. You can change it by going to the super secret settings.
Jay Be
Press ESC and click the "Super Secret Settings" button, until it comes back to the way you like it, or just simply restart your client.
Armadil
Press F4 to remove all texture-changing effects (shaders).
Patrick