Kola Colored Lens and Flash Filters ($15; Photojojo) come in a pack of eight. While not iPhone specific, the colored gels can be held over the lens or flash of an iPhone or iPod touch to create a range of colored effects. The filters are a great old school tool for adding a tint of a single shade to your photos, but they’re not ideal for layering and there’s no way of actually attaching the filters to the camera.
While Kola offers a wider range of color, a hands-free alternative to the filters can be found at USB Fever for iPhone 3G and 3GS users. Here, you can get a single lens that rotates to display yellow, green, or red filters ($24).