Shane Hurlbut explains how to use color correction gels in subtle ways to create color contrast with the new digital sensors:
Back in the day, CTO was the only way to warm your lights up in increments. You had: 1/8, 1/4, 1/2 and Full CTO.
In the mid-nineties, Rosco introduced a warming gel called CTS, which had more yellow than orange in it. I prefer yellow over red and orange as a color on skin tones. The CTO line seemed to deliver more red and orange. With the digital sensors, I find that the CTS works the best on skin tones, giving that vitality and golden glow.
When I use HMI lights, they all tend to be a little cold to my eye so I warm them up a little bit to start. Mornings are more calm and clear, which gives you that greener, cleaner look at sunrise. Sunset is a more an orange feel and tone, mainly because all the particles that have been stirred up during the day add a warmer feel. Obviously this is just my personal observation. I prefer to use the best aspect of CTS and CTO and not just one gel for all looks.
Read the rest of the excellent detailed tutorial here.