The colour with in.

Its possible to create colour images form black and white film by using RGB filters and creating 3 images, this gives you 3 tonally different images and this is the key to adding the colour.

 

In Photoshop add a new layer and fill it will;

Red - cyan 

Green - magenta 

Blue -yellow

Then change the blending mode to screen.

Merge each image and its colour layer the stack all the merged layers. Change the blending modes of the top 2 layers to multiply.

I never acomidated for the reduced exposure due to the filters so my image came out dull, with a exposure lay I managed to boost the colours to look realistic.

To understand how this works you need know the relationship between RGB and CKYM.

RGB comes from projected light by mixing the 3 colours you can produce white

CKYM comes from print by combining Cyan,Yellow and magenta you produce black 

They are Also contrasting colours so the red filter only records the red shades on to the negative, when converted to positive the colour is also inverted giving  Cyan.