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Plastic. There are a number of ways you can approach this, but regardless of the method, you'll be wanting some sort of stiff, rigid plastic, as thin flexible plastic poses too many distortion problems (unless you're into such surreal effects, in which case, go ahead.)
If you can get colored plastic or theater gel, then you're pretty much set. If not, or if you want to create custom color effects, then you can use clear plastic and then color it later. A great source of clear plastic the size of your lens is actually from disposable cameras.
Color. So how do you get color on a piece of clear plastic? The crudest way to do it is with something akin to a Sharpie pen. Another neat trick is to actually use watercolor, which is easy to manipulate and blend for a variety of color effects—and best of all, easy to remove, meaning that you can reuse the same piece of plastic for different effects between shoots. If you want grainier color effect, you can draw on the plastic using crayon or pastels. Feel free to mix up your colors for rainbows, gradients, even small designs to be overlaid over your shot.
Scissors/Box Cutters. Depending on the hardness of the plastic, you'll need something to cut it to size with.
Supplies for filter mount. There are many techniques for creating a filter mount, which will vary by camera. Please read through this article on creating your own DIY filter mount to find the appropriate filter mount for your camera.
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Your first step is coloring the plastic. Use whatever sort of coloring method suits, as previously discussed. Once you've got this complete, select the region out of the colored plastic that you'd like to use for the filter, and cut to fit whichever method you'll be following below. Measure twice, cut once! While plastic is easy to come by and coloring hardly takes a long time, you might not appreciate having to repeat this procedure too many times.
But how will you be cutting this? If you're just planning on holding it up in front of your lens every time you shoot, then just make sure it's large enough that it fills the entire viewscreen of the camera at whatever distance you're planning on holding it away from the camera. Play with this ahead of time to see how in or out of focus you'll want the details of the color filter to show up.
If you want this properly attached to your camera, it's time to make yourself a proper filter. As previously stated, please follow the instructions on this article on making a DIY lens mount.
If you want the color filter placed in front of the flash, the easiest method is just to tap it in front of the flash, or hold it up yourself while shooting. That's it!
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Using The Color Filter
There are a number of ways in which you can use the colored filter.
If you've made one that is placed in front of the lens, then just attach and shoot away. Depending on how you've made the color filter, you may want to play around with aperture to see how in or out of focus you want the color filter itself to be. A low f-stop will result in the color filter being out of focus, evening out the color, while a higher f-stop will make more of the picture, including any unevenness and texture to your color filter, more distinct and in-focus. The color effect will show up on your viewscreen, so you'll be able to easily adjust beforehand manually.
A different technique is actually placing the plastic in front of the flash. This will not only even out the color in your filter so that any unevenness isn't quite as apparent, but also allow you to play with a variety of flash effects, according to your lighting needs. The colored plastic will dim the flash somewhat, so be prepared to adjust exposure for that.
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Feeling creative? If you're into scratch textures in your pictures, you can always etch your piece of plastic as well as applying color effects. This can save you a few steps in your digital editing software later on.
Otherwise, don't be afraid of trying out rainbows, color gradients and other color filter effects to a variety of situations. The intensity of the color in the filter can also effect the image. A subtle increase of blue in a photo, for instance, can make the whole photo feel colder. Check out this article for more information on tones and the moods color creates in photographs.