There are a few ways to make an image dull in Photoshop. One way is to use a Dull filter. You can find this filter in the Filter menu. The Dull filter has three options: Bright, Mid, and Dark. You can choose which option you want to use by clicking on it. The Bright and Mid options make the image look brighter and more vibrant, while the Dark option makes the image look darker and more muted.