First, there are two problems with lighting value and color. If your lighting on your canvas is very bright, then you will tend to paint your values much darker they should be. And if you are painting in the dark - well you simply can't see in the dark unless you're a cat or something. So how bright should your light be? I would have enough light so you can see what you're doing clearly, but not much more.
What about color? Now that is a different problem.
For portrait work and still life, I always try to have the same light on my canvas as what I am painting. I try to start and finish portraits from life; I do not trust photos when it comes to color. North light is best because throughout the day, the color is, for the most part, unchanging; and I usually block off the lower half of the window so that the light is always coming from above.
Since I cannot always work only when the sun is shining, I also have a set of fluorescent 4' tube lights mounted above the window. I use the GE Chroma 50 bulbs, which are a D50 white point with a white diffuser cover.
With this setup, I have very little color shift from the natural light to the artificial light. I also use these lights on cloudy days. There is really no such thing as daylight bulbs, since sunlight runs from cool in the morning to warmer towards evening. So unless the manufacturer gives the white point of the bulb you do not know what their idea of daylight is. I ordered a set of so called daylight bulbs and they were so awfully warm that every thing was like painting by sunset. I even find the normal 4' tubes they sell for shop lights are better then a lot of the so- called daylight bulbs.
Now for illustration work, I have a different problem. The final work is the reproduction, not the painting, and I need to know how the repro camera will capture these colors. So for that, I have a 3200K incandescent photo lamp. I actually have two of these, since they are also what I use to photograph paintings with. Now I don't paint by this light; it is way too bright and my finished work would be much darker than I want. I just turn it on the painting to check from time to time, as to how it looks. This is getting to be less and less a problem, since more and more of my illustrations are painted digitally, and my color calibration is on the monitor
More on natural lighting, and the problems of plein air painting.
You don't always have perfect north studio light. Sometimes you will be painting on location, or outside, where you can have reflections coming in almost every direction. For the most part, if the lighting on your subject and the canvas are the same, and you start and finish within a 2-3 hour time frame (or paint at the same time of day), you can paint in any light and get a nice painting. But sometimes things can go wrong due to several factors, such as environmental color. One example is if you are surrounded by green leaves all that reflected cool green light could cause you to over-warm the colors in your painting to compensate. Or another example, and a more common one, is your subject is in one light and your canvas and palette another; for example, if your subject is in a window light, but you have an incandescent lamp on the canvas and palette.
Outdoor painting can be a lot of fun, as well as a challenge, due to the ever-changing world of nature. You have only about 2-3 hours before the shadows and quality of light change too much to continue working. I even would not go much more then one-hour sessions. Although it may seem obvious, don't try to finish a painting started on a sunny day, on a cloudy one; or one started in the morning, in the afternoon, or vise-versa.
Plein air painting and the academic classical style of painting can be at odds at times. Although I love the works of William Bouguereau I sometimes feel some of his outdoor scenes seem to have indoor studio light on the subjects (cool highlights with warm shadows). It is this very fact that I feel had the most influence on the development of the impressionist style and their departure from the academic style. Although an impressionist style is very suited to plein air painting it does not mean you have to paint that way to paint a outdoor scene. And of course, we have the camera to help capture the fleeting effects of nature to use as reference. I even know of landscape painters that always paint in their studio from photos so you don't have to subject yourself to the many down-sides of plein air painting - even when your subject is the out of doors.
How much light do you need on the canvas when you are painting?
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