Squint to see the three value zones. At first it won't be easy. By adjusting how tightly you squint your eyes you will be able to discern subtle value differences.
The easiest value to see is the dark. The next easiest value to see is the light. Things get complicated in the middle value range but be persistent and you should be able to see the middle value zone as one flat shape. It's tempting to paint value shifts within this middle range. But force your values to be similar, making any value shifts very slight. Concentrate on keeping the middle value colors very saturated. Emphasizing color saturation will help you keep those values anchored in the middle zone.
In the accompanying video I have added light accents and dark accents into the building. If you see accents paint them in. But remember that they are accents and should not dominate or over power the basic middle value. Especially in white shapes you will notice a wide value range. But even with white objects try to keep the values as close to the middle value as you can. Don't use pure white with highlights.
This concept —though somewhat difficult to internalize—is critical to painting successfully in the field. I hope you find it useful. Let me know how it works for you.
Brad Teare — June 2014