In the course of this tutorial we will employ a destination blend effect container with a desaturate inside, a chroma key effect and a negative alpha effect, plus make use of the source opaquity media track variable in order to achieve a saturated strawberry in a grey scale photo.

If you'd like to use the picture to try this out, download here.

Step 1
Import applestrawberry.jpg.


 
Step 2
Get a chroma key effect from the pixel filters/keying filters f/x folder and drop it onto the applestrawberry.jpg track.  On the current tab, depress the source crop rectangle toggle and select only the area around the strawberry to limit the key.


 
Step 3
Depress the colour picker toggle and select a square sampling of the blueness surrounding the strawberry.  Then adjust the y, u and v tolerances accordingly. (The y, u and v values indicate the key colour) Note: if one were to have a not so solid background colour, one may choose to use 2 chroma key effects to key out the red and green parts of the strawberry instead.


 
Step 4
Get a negative alpha effect out of the pixel filters/pixel channel ops/channel inverters f/x folder and drop it onto the applestrawberry.jpg track.


 
Step 5
Now the alpha channel is reversed, which has the effect of turning almost everything transparent.  To make only the strawberry transparent, we will need to copy the effect rectangle variables from the chroma key and paste them on the negative alpha effect. To do this, select the chroma key track and copy the variable settings at the bottom of the vars panel (as of v.0.934 there are no source rectangle copy/paste icons so we're making it quick by copying all the variables).


 
Step 6
Select the negative alpha effect track and paste the variable values in at the bottom of the vars tab.  This works in this case because the negative alpha effect only has left, right, bottom and top crop variables but note that the copy function here copies ALL variables with key frames.  Now just the strawberry is fully transparent, making it appear black because nothing is behind it.


 
Step 7
In order to desaturate only the parts of the image that are NOT transparent, we can put a desaturate effect into a destination blend effect container.  Get a destination blend container from the effect containers f/x folder.  Drop it onto the applestrawberry.jpg track.  Also grab a desaturate effect from the pixel filters/simple filters folder and drop that on the destination blend container.

The destination blend container writes its contents only to the opaque areas of the parent, so the expected result is that everything turns gray scale except the strawberry which remains transparent (for now).


 
Step 8
In order to bring the strawberry back to life, select the applestrawberry.jpg track and its vars tab.  Hover the mouse over the output opacity variable strip (top) and press the up arrow on the keyboard (or failing that, click at the very right edge).  Now the track is fully opaque which brings back the original colour of the strawberry.

This works because the output opacity is calculated after all the effects.


 
If you have any questions or comments, leave 'em below.