Revised concept

Two characters
Two worlds
One with colour
One without
The colour world is on top and the black and white world is on the bottom. (above and below ground)

I've decided to make this more interesting by making 'colour' an actual substance in these worlds, so 'colour' comes with weight (so its quite a heavy substance).

Main points to get across:
communication
co-habitation
people are never alone

Here is the storyline.
The story starts showing the two main characters in their worlds. It shows them day in day out maintaining their worlds. Due to the weight of the 'colour' it eventually drains off the buildings and into the ground where it sinks down to add colour to the black and white world.

So the colour characters job is to maintain colour in its environment (city) and the b/w characters job is to maintain the monochomatic landscape in his environment (foundations of the city).

Both characters feel quite overwhelmed with these tasks as the job is never finished.
Finally, the black and white character breaks down. It can't take it anymore. So it in an effort to release its frustration over the situation accidentally pulls on one of the smaller foundations to a building. This building being smaller than the others fell down into the black and white world which in effect opens up a void that acts as a drain for the colour to fall into the b/w world.

After discovering and going down/through this void/hole, the colour world character discovers that there is a whole new world beneath its own - in black and white.

Meanwhile the b/w character is in despair over the sudden influx of colour that's contaminated its world, thinking that its all its fault it doesn't know what to do and so just sits as the colours that were drained attach to him, weighing him down.

The draining of colour in the colour world has also cause the color character to be devoid of colour as well, most of its colour was dependent on the world of colour too. But as it travels though the b/w world (which is now completely coloured) it seems to attract the colour that had fallen on to the b/w world, soaking the colour back into itself. Meandering though the b/w world, it eventually finds the b/w character slumped on the 'ground'. Reaching out to touch the b/w character wondering what was wrong with it, the colour that had fallen on to the b/w character is immediately soaked up and absorbed by the colour character.

Realising that it didn't feel weighed down anymore, the b/w character realises that the colour character was there next to it and that that character is fine with the added weight of the colour.

Realisation dawns on the coloured characters face as it begins to understand what has happened. The coloured character makes gestures and notions the b/w character to follow it.

Both characters go back to the coloured characters world and realises that the main reason the colour kept draining out was because the colour was so heavy it seeped through the ground.

Through communication with many gestures and sounds, they decided to brake down the existing ground/barrier and build a new one out of both colour and black and white. One that was strong enough to keep the colour in the colour world although they keep a way - perhaps a trapdoor? as a permanent link between both worlds.