Hi

You need a black and white mask of the rose to create the effect.

With video on the timeline this can be tricky in maintaining mask alignment, with still images on the timeline it is easy.

The image below shows how this can be done.

1.  Export still image from the timeline and save as jpg.

2.  In an image editing program select the rose and change its colour to white

3.  Invert the selection and change the rest of the image to black, save the jpg.

4.  On the timeline duplicate the object to which you want to apply the effect and place it 2 tracks below the original.

5.  Import the saved mask and place it on the track between the two objects ( see image) and apply the Alpha Effect to it. 

6.  Apply a Saturation curve to the upper image.

7. Check you get the correct area turning black and white - if not click the arrow at the beginning of the mask so it points the other way.

Here is a capture from further along the timeline

The whole process took about 5 mins to create

HTH

John