It's much more fun than that. The colourist has the ability to change
any colour into any other colour, a process known as secondary colour
correction.
It's an artistic skill requiring good colour vision and good judgement
alongside the ability to manipulate complex controls in real time often
with directors sitting behind you applying pressure and irritation.
For a TV show, as Dave Plowman says, the colourist will usually just
fix the colour changes due to lighting - there is a budget after all.
For a movie or an advert many hours will go into getting the colours
just right, you'd be amazed how picky the directors can be.
The real kit is expensive and specialised but very quick to use and
very powerfull. If you have access to Adobe Premier (free with some
Sony laptops) you will find a less powerful version hidden in the
colour controls - you can waste hours playing with it!
HTH
Calvin