I'm guessing you want to do OSL (open source lighting)
It's pretty simple once you know where the light should shine.
The light source should be painted to a nearly white colour.
Then paint the darkest shade of light at the furthest reaches that the light will fall at.
Begin to work your way back towards the light source using a lighter shade of the darker colour, getting brighter and bright until you reach the main light source.
Here's an article by a guy called Shaun on coolminiornot, he,'s helped me with OSL in the past.
http://www.coolminiornot.com/articles/1328-oslOr this
http://www.coolminiornot.com/articles/1106-painting-lighting-effectsCheers
http://quests-of-legend.weebly.com