This is known as a Color Pass. Here's a post I made from the archived Avid Liquid forums (06/11/2006):
What you want to do is use the Color Correction editor
and use the Secondary color correction tools. In particular you need to
look at the Six Vector Color Correction tool and the Selective Color
Correction tool.
Start with the Selective Tool. This allows you
to pick up to 15 specific hues. You can also adjust all the colors for
everything not selected. So, let's say you want to keep the red and
yellow flowers and make everything else B&W. You can select the red
and yellow of the roses, and the desaturate the color for everything
else.
This is all described in chapter 13 of the Liquid 7 manual
(it starts on page 776 of the 7.0 book; I haven't looked it up in the
7.1 manual). The manual is included as the PDF under Liquid's Help.
The short of it is:
- Load the clip into the color correction editor
- Expand the CC dialog to display all of the controls (it's the triangle icon at the bottom middle of the dialog)
- Click
on Add Selective Color. This activates the pipette (color picker tool).
Click on the color you want to isolate. You can also drag to choose a
range of colors. Holding down the control key lets you add to the
colors as well, so you can use Ctrl-left-click to select colors. The
colors here that you're selecting will be averaged, so for example you
want to get all the ranges of red.
You can see how well you're
picking by selecting the first color, and going down in the Selective
tool and changing the Cr or Cb to the highest level. This will change
the color of what you've selected, so you can go on and select the rest
of the range for that color. It will also show the trouble spots
(colors on thing you don't want selected that are real close to the
selected color). - Fine tune the selection by changing the Color
Isolation L, S, and H parameters. This may remove unwanted color from
the selection.
- Set the Cr and/or Cb back to normal.
- Click
on Add Selective Color again and repeat for each color you want to
isolate (so do once for the red flowers, and then again for the yellow,
etc.)
- To work on all the colors you didn't select, click on the grey area around the color wheel.
- Reduce the saturation (S) until the rest of the scene goes B&W. The flowers should remain red and yellow.
What
you'll probably find is that there are colors in items that you didn't
want selected but for which you couldn't isolate. For these, you have
to use the Keying editor. Put a copy of the clip in the track below
this track, and turn it to B&W. Now on the top track, add the
Classic Keying Editor and key out the problem areas. This will allow
the B&W on the track below to show through. Note that the keying
editor is keyframeable, so you can change the keys to follow the action
in the scene.
Color passes can be difficult if you pick the
wrong colors to work on. For example, it's very difficult to isolate
pale reds, pinks, yellows, orange, and brown as these are normally
found in human skin tones. It's much easier to isolate vibrant colors.
I hope this helps.