Compositing

Add light to shot in photoshop

(Rendered in maya)

Take the shot into photoshop, create a new layer, add a gradient and set the layer to multiply
To emphasise any lighting in the scene

Change Colour of object after rendering

Render out a RGB version of the object and it will create alpha masks of the object
Create a selection mask of the object
Create an adjustment layer with hue/saturation
Alter colour to whatever you want
Useful if a client comes back wanting some colours tweaked, or the levels, whatever it may be. Just render out alpha masks.

How to render a character separate to background

Select character > create layer from selected (in render panel)
Attributes > turn off primary visibility
(To turn off multiple object at once, select them all > general editor > attribute editor > render tab > select primary visibility > 0 (to switch off))
Right click on the render buttons on the little render window and you can render from different cameras
Render settings > Quality > Turn on FinalGather
Some of the advanced settings are hidden > scroll down > Final Gathering Accuracy and Point Interpolation are the most important settings to play with
Note you can increase the lighting quality without changing render time too much, bumping up the quality increases render time exponentially

Compositing

Black Light