If you have problems to determine the exact white balance in a mixed light environment, there is an easy way to measure white balance. One of the commercially available solutions is the expodisc (something looking like a filter you put in front of your lens), giving you an 18% neutral reference. The only small issue with such an expodisc is the price. Not an issue if you are a pro user because it is a guaranteed reference, but a bit expensive for the occasional amateur.
The official expodisc site can be found here: www.expodisc.com
Tom Hicks, a extraordinary nature photographer, active on www.fredmiranda.com, has designed a compatible solution, using cheap components. I tried it for myself, and this is working pretty well.
Basically, you put white neutral material between two cheap UV filters, with an optional adapter ring if the filters do not fit on the lens. I chose two 77mm filters (the most common size on my lenses), allowing to adapt to almost any size with the appropriate adapter rings. You install that combo on the lens, measure white balance, and you're all set.
The tricky part is the selection of the neutral material, I first tried with the foam I got with the filters I used, a coffee filter, but both had a yellowish cast, giving slightly bluish pictures (slight blue cast). The best material is the material used for masks (something you buy in a DIY shop and typically use when using chemicals or painting).
Here is the simple procedure to be followed:
The components
The Hickspodisc (this was with the wrong material with a yellowish cast - to be avoided)
The type of mask recommended to get a neutral white material - just remove a layer of the material used in the mask and you'll have the perfect expodisc.
A picture shot with white balance on auto
The same picture with the improved white balance
