On the Apple Discussions forum for Color someone asked about isolating several colors simultaneously while de-saturating the rest of the image. There are two approaches. The first involves using the Saturation Curve in a Secondary room. The second approach, as I answered on the message board, is to use the ColorFX room. What follows is a more detailed explanation of how to use the ColorFX to accomplish that goal – I’ve even included pictures (click on a image to open it full size).

So, here’s the initial image (just happened to be up on the screen at the time I decided to write this posting):

We’ll isolate 3 different colors; blue sky, red building, yellow equipment. The rest of the image will be Black & White.

Here’s the final node tree to create that result (click for a full screen image):

The first thing we do is pull three different keys using the HSL Key (for detailing instructions on how to use any of these nodes – check out the user manual available from inside Color under Help > User Manual). In this case I’m using the nodes HSL Key3 and HSL Key to pull the red and yellow elements. Then I combine those two into a single image using the first Add node. The output of that first Add node looks like this:

Next we use another HSL node (HSL Key2) to pull the blue sky. We combine that key with the initial Add node using another Add node (labeled Add2).

It’s output looks like this:

One thing to keep in mind when using Add nodes… they have Bias controls which are initially set at .5. This means it’ll add the sources feeding it at 50% of their initial values. If we leave them at these settings, we’ll end up with alpha channels at 50% intensity. We don’t want that, so we need to set them to 1.0 (or 100%) like this:

Next, we need to make our desaturated background. Back on the ColoFX tree you’ll see the B&W node feeding a Curve node – this is to set the final look of the background image and gets fed into Source 1 of the Alpha Blend node. The Source 2 input doesn’t have anything feeding it, so it defaults to output of the Primary In + Secondary rooms. The third input is the Alpha input and we feed that the output of the Add2 node (which, I’ve softened with a Blur node).

Here’s the end result:

That’s it!

- pi

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