Trouble replicating the animation where the colour only spreads onto a specific area I'm trying to achieve this effect in this video: https://www.youtube.com/watch?v=2-6Xz4tWigM&amp;t=49s

Animation is at 0:47.

I'm reletively new to the node editor but understand its mechanics but a noob to masking. 

I believe a mask could be used to affect only the two characters in the image but i don't know how to achieve it in the node editor. 

My process so far: add image node, which ive cropped and transformed to get the image in position so i can see it in the viewer.
Create Mask of boy on the left in the UV image editor using ctrl click and ctrl c to finish making the mask. Add mask node and select mask.
Add Movie clip of an ink drop and changed its colour to red.

That's it so far, completely stumped 

Any help would be greatly appreciated.
Even though you have part of the process completed, I will give an overview of the steps.
You could transform all images in the node editor, but since Blenders transformation nodes are inefficient to handle, I will suggest the 3D space.

Load your image into the compositor. Set the project size to match it's dimensions.
I will use the images as planes addon to quickly get my ink movie clip into the 3D view. Import it with the options Auto Refresh and Shadeless.
Then I added an orthographic camera. The movie clip should play, if you selected auto-refresh.
Set the image as a background image in the 3D view. Transform the movie clip plane in camera view.
Set the World Background to black. The render will be used as a mask based in the luminance.
Mask the image in the Movie Clip Editor.

Load your Renderlayer and Mask into the compositor and multiply them with a ColorMix node.
To color the masked portion red, use a ColorMix node. Connect the multiplied masks with the Fac input.

Rendering it results in the desired effect.

Further notes.

If you change your scene size (using the percentage slider), you will need to add a scale node to the Input Image.
If the ink movie clip mask is not white, the effect will not be 100% opaque. You a RGB Curves node or a colorramp to shift its whitepoint.
You can use the mask with many other nodes other than a MixRGB node.