Never think you have to get exactly the colours you need with a render, or else you will be just wasting render cycles. The area of 3D image creation known as "Shot Finaling" is comprised of both lighting and compositing (or post processing) collectively, they really go hand in hand. Lighting should take your image about 75% of the way in my opinion. The last 25% of the work is fine tuning your colours and other effects in a 2D package. This just makes your time more efficient. Every image I make in 3D always has some 2D work done to it, some images have extensive 2D work, and look very different from the original 3D render.
Consider this, you want more of a saturated blue in the image that just took you 4 hours to render. Do you:
a) adjust some settings in your 3D package to try and get the result you need, then wait for another 4 hours.
or
b) take your image into a 2D package, create a quick mask and adjust saturation as needed, giving an almost instant result.
I never try to perfect my image from the 3D render, it wastes too much time. I get the colours, shape and form almost there, then spend the rest of my time adjusting it easily in 2D.
