I was reading a post and replies about outdoor fill light on cinematography.com this week. Someone posted a question about how to create natural looking fill light while filming outdoors. Their question was mainly about how much light, rather than the characteristic of light. However a number of people chimed in about how important it was to maintain a natural color in the fill light.
Just so we get the first question out of the way, a few people recommended to keep fill light about 1.5 stops down from direct sunlight to make it look natural enough.
Now, as for the light quality there is an additional parameters to discuss. First, if your actor is back lit...which they probably are outdoors, then the fill light will be coming mainly from ambient sky light. If mid-day direct sunlight is 5600K, then sky light might be in the neighborhood of 6500K to 9500K depending on the time of day. An HMI light may or may not get you close, however both Lee and Rosco make blue diffusion materials that can help compensate for the difference between an HMI/LED/Plasma light and the much blue-er sky light (click on the picture above to see some of the materials I found as examples).
The other thing to consider is reflected light off the ground. This is where fill light might become green due to grass or brown due to dirt. In an interview with Natasha Braier, ADF in American Cinematographer she talked about how they ran their diffusion rags through dirt and coffee for The Rover. She wanted the world they were creating to feel dirty, so even the reflected and diffused light was dirty. If you've seen The Rover, then you know that the lighting looked consistently natural through the entire movie.
This is now something I'm going to keep a watchful eye over, i.e. the color of ambient and reflected fill light in a scene. Only in comedies does it make sense for everything to have eye popping clean light. In the real world that doesn't exist. The next trick is how to explain to the producer that I want to take their $300 diffusion rags and spill coffee grounds on it.