its really easy m8!
- Open your image, and go to "layer"
---> "new adjustment layer"
----->"channel mixer"
-------> click ok,
---------> tick the monochrome check box.
That makes a new layer and turns the image black and white.
---> select your standard brush tool (or press "b" on the keyboard).
-----> set the foreground colour to "black"
-------> and the background to "white"
---------> then just paint over the parts of the image you want to make coloured.
You have to zoom in and adjust the brush size to get the neatest line possible.
Its as simple as that! Enjoy!