How To Use RGB Filters On Grayscale Images
If you’re working on grayscale images, you’ll find there are some Photoshop filters that won’t work (they’re grayed out, so you can’t access them). Of course, it’s always the really cool filters, such as Lens Flare and Lighting Effects, that are grayed out. But don’t be dismayed (in fact, be “mayed”) because you can still use those filters—just switch to RGB mode (it’s found under the Image menu, under Mode), apply the filters, then switch back to Grayscale mode. It won’t affect the color of your image because, well, there is no color—you’re working on a grayscale image. Switching to RGB doesn’t suddenly pour color onto your image; your grayscale image will still look grayscale in RGB. When you switch back to Grayscale mode (after applying the filters), you’ll get a warning asking, “Discard color information?” You can safely click OK, because after all, there was no color to begin with.