Partially colourize greyscale video

[Jenni5EAF]

Hi all,

I need some advice on After Effects please. I am trying to gradually colourize one thing in a video which must then go back to greyscale. So my first idea is a drop of water rolling off a leaf and dripping down. The drop of water must be in colour and perhaps a small halo of colour around it as it rolls down the leaf. But the leaf must go back to grayscale once it's passed. Then it drips right off the leaf in colour and the leaf is totally black and white again.

Any advice or tutorials on how I can do this please? I've managed to find some on Youtube, but they're not giving me exactly what I'm looking for. I've attached my results so far, but I need something cleaner. 

TIA

[David Arbor]

For something like this, you might be best off rotoscoping the shot, i.e. manually keyframing a mask.

1) Create a mask and animate it around your water droplet.

2) Apply an effect to desaturate your clip.

3) Twirl the effect open in the timeline (you can also double-click the effect name in the Effects Control panel).

4) Twirl open Compositing Options.

5) Click the + button.

6) Choose your mask. This will then mask your effect so only the water droplet will be desaturated.

7) On the mask, click the Invert button or change the mode to Subtract so the droplet is in color and the rest of the shot is desaturated.

8) Feather the mask a little to add the small halo you mentioned.