April 21, 2011, 2:06 p.m.
posted by sunlight
Expose Only One Color
It is rare you will ever want to remove color from a shot, or an entire movie. Yet, in some instances, it makes sense within the story you are telling. Using the following technique, you can create a black and white scene, while allowing one color to "pass through," known as a color pass effect. Figure shows an original and completed image using the color pass effect.
Using yellow to help alert people to avoid physical harm
When planning on using this technique, it will help if you can limit the color you would like to expose to the object where you would like to draw attention. For example, if you would like to focus on a girl in a red dress, you should try to avoid having other red-colored objects in the frame.
Layering Your Video
In order to create the color pass effect, you will need to create two layers of video. One will contain the image as a black-and-white composition, while the other will contain just the image/color you would like visible. When combined, they will create single image with only one color visible.
The method you use to create two layers will depend on your editing system. For the majority of systems, you should be able to drag-and-drop your footage onto your timeline. You can then repeat the process; however, you should drag your footage either above or below your first layer.
Creating a Black and White Image
The first step in creating the color pass effect is to completely desaturate your lower-level track. This will create the base image for your composition as black and white and allow you to work more efficiently. To desaturate your image:
Tools Effects Palette Image Color Effect (Chroma Saturation)
Effects Video Filters Image Control Destaturate
Even though you've now removed all of the color from your footage, you have affected only the lower track. Therefore, your image will still appear in your record monitor (a.k.a. Canvas or Program View) in full color. Don't worry; that'll change soon enough.
Choosing a Color
The next step is where you choose what color you would like to keep. You can do this in a number of ways, but using either a chroma key or a color key is the easiest. You should apply this effect to the top layer of your composition:
Tools Effects Palette Key Chroma Key
Effects Video Filters Key Color Key
Some systems enable you to simply click on a point of your image to select the color you would like to use. If not, you will need to use the controls provided to locate the color you want to expose. As always, don't worry about being exact because you'll be able to change any selections you make later in the process.
Exposing the Color
After choosing your color, you might notice that you accomplished the exact opposite of what you intended. Instead of exposing the color, you removed it. Fortunately, you can invert the key, thereby exposing the color and removing everything else. You should select the clip and effect/filter you've applied, and click on the invert checkbox.
After inverting your key, you may notice a few spots in your image where the color exists, but you don't want it to. For these spots, you can simply crop or mask them out of the image.
Hacking the Hack
Another approach is to maintain color throughout your image, but to emphasize a specific color. To accomplish this effect, instead of converting your image to black-and-white, you can reduce the saturation on the bottom layer and increase the saturation on the top layer. This will cause your chosen color to subtlety stand out from the rest of your image.