The worst kind of lighting is dappled shade or uneven lighting. If you set up under the shade of a tree, you may see this happen. Little sunspots will fall on the surface, and they will throw off your perception of value and color intensity. The same color in shade will look much different when a sunspot hits it. If you try to adjust the color so it looks the same under both light and shadow, once back in the studio you'll be surprised how it again looks so different everywhere. Here's a dappled shade painting.
The next worst is full sun. You'll think your color mixtures are all too bright, and consequently you'll dull and darken them to get the effect you want. I guarantee that when you bring the painting into the studio, it'll look all muddy and dead. "But it was so rich and full of color outside!" you'll complain. With experience, it is possible to learn to compensate for this, but it's still a struggle.
The very best situation is gentle, even shade. It's much easier to get accurate color relationships, and the painting should look about the same in the studio as it does in the field. I've found the best way to get this shade is either to point your easel into the sun or to use an umbrella. Pointing your easel into the sun, of course, doesn't necessarily point you at your chosen subject. Sometimes, your subject may be off to to the left or right, unless you paint backlit scenes. You can work with your subject off to one side, but if you find yourself whipping your head around like that little girl in The Exorcist, you should consider using an umbrella. A pulled neck muscle is no fun.
Watch out for "sun creep." I've seen many students start off with some good shade on their canvas, only to have the sun creep around and start throwing a raking light across it. Sometimes the top bracket of the easel or a tree limb will throw a shadow down. We get so involved with the painting that we are often blissfully ignorant of such changes. You must be as aware of what's happening to the light on your canvas as you are to what's happening to the light on your subject. Here's the painting with "sun creep."
If shade is such a good thing, can you have too much of it? Unfortunately, yes. Avoid painting on big wrap-around porches or under trees that cast a deep shadow. You will be blinded by the lit landscape beyond, and your painting will look like a dark silhouette in front of it. Sometimes this is all the shade you can find, though, and the trick is to angle your surface so some light from outside bounces onto it.
Finally, don't wear light-colored or brightly-colored shirts, especially if you're oil painting. If any sunlight hits your shirt at all, a light-colored fabric will throw a glare on the wet paint, and it'll be impossible to judge values. Bright colors are bad, too. If you wear a red shirt, it'll cast a red glow over the painting, distorting the color of your paint mixtures. (Although glare isn't a problem for pastel, this bounced, colored light is.) I've seen many painters, professionals included, wearing white tee shirts in the field. I'm not always thinking when I dress in the morning, and so I've made that mistake, too. I always end up boxing with my painting - "bob and weave," as the trainers say - to find an angle where the glare doesn't get in the way. Here's the painting with red cloth in front of it and then with white cloth.
By the way, if you're thinking of joining us for the Grand Canyon painting retreat (April 26-29), you need to sign up right away. I learned today that lodging is already filling up. Trina and I made camping reservations with no problem last week. You might consider camping - it's a great way to get close to nature, and the Mather campground is really quite nice. Here's a link to camping details. Here are details to the workshop itself. I have only a couple of spots left.