Several reasons for this.
1) Warm underpainting, some of it might show through to the final layers, warm brown is a nice inoffensive colour that will not disturb anything, compared with say, lemon yellow underpainting.
2) Brown is a very cheap, very permanent earth colour. Some earths like Burnt Umber are amongst the fastest drying pigments available to oil painters. If you want it dry the next day, warm brown is a great choice.
3) Brown is usually a very dark orange, most Caucasian people are a very neutralised orange. It's a decent starting point for pictures of white people, assuming no crazy coloured lights.
btw, my initial answer to you may have come across as slightly flippant, but 90% of everything useful you learn about painting will happen when you are actually holding a brush..
Ah...you're talking about what color to make an underpainting? And as usual it depends on where you're going with it. Simply searching on "underpainting" turned this up: http://en.wikipedia.org/wiki/Underpainting
It should give you some good leads for further research.
I mostly paint alla prima with opaque pigments so I start by sketching the general shapes in in whatever colour is not going to show through. Usually yellow ochre or cerulean hue because they're cheap and light.
My beginning painting teacher says, if you are painting with oils, you can start anywhere... because you can always correct it and paint over. This is the advice he gives to total noobs who are afraid to start.
As you get better and learn more, there's a lot more method to the madness, but when you are just starting start anywhere and start correcting. Most of the learning occurs when you are making corrections and adjustments - not when you accidentally do it just right.
General steps we did in my beginning painting class last semester, he had us do the underdrawing correct first, then start laying in some general values. First painting we did was black and white, so we can concentrate on seeing the values. Second was using the imprimatura layer. Again, good underdrawing, then imprimatura layer, with lights and darks, then we started adding color.
Third painting we did using more of ala prima methiods (not a real ala prima, where the whole painting happens in one sitting) but without using layers of paint and glazes, but going for the actual look right away. (I'm not sure what the correct technical term is for it. The teacher called it alla prima, for our purposes, but more advanced painters make a distinction between the single sitting or on location painting, vs something you work on few hours at the time, at home or in your studio.
Initial under-painting we called imprimatura in class: http://en.wikipedia.org/wiki/Imprimatura
Most often, in classical painting we are taught they used Ocher, Burnt Sienna, Raw Sienna, Raw Umber and Burnt Umber, and is what we were asked to start with in class. For our beginning level, we were asked to make it around mid value, and what hue to use would be dependent on the general tonality of the subject. Also, you can mix the above to get the hue and value you want. On the long run, as you start using it, and learning how it behaves and how it affects your painting, you can vary things to achieve deliberate effects. On the long run, the teacher told us, you can use any color you want.... but they're not all going to give you the same results.
Last edited by Conniekat8; February 23rd, 2012 at 12:40 AM.
For me, one of the first things that I do is to tone my canvas (I hate the white little dots that show through). This also relieves me of the issue that I don't know where to start on the canvas to place my first set of marks.
When toning the canvas, I typically use burnt sienna just because I always want a warm undertone. If I'm painting outside, I'll tone the canvas with the general sky color.
Once the canvas is toned, I start laying down some lines (thinned with walnut oil) to indicate general positions of my objects. For me, the detail comes later. I just need to block in the positions. Once the lines are down, I block in the various mid-value colors of each area. So, by the time that I'm done with it, I have a general massing of my colors and my masses.
This is a matter of practice. I used to do a lot of value paintings. Color comes later. No matter what color you use, if your values and drawings are wrong, the color isn't going to save you.
If you want to learn to get the right color, create a color chart of your mixtures and hold the color chart up to each part of the setup. Takes time and you'll get faster, but it'll show you the colors that you mixed to get an approximation of what you see.
You don't HAVE to start with an underpainting (and, as jeff says, there's a gazillion different ways to approach underpainting depending on what you're trying to do... For instance, Maxfield Parrish had his own weird multicolored approach that looks like a four-color separation, bright red under the green areas, bright blue under the brown areas, etc...) And everyone has their own ways, that's something that will most likely evolve as you progress...
Really, just start somewhere. Anywhere. Trying simple value studies in two colors sounds like a good idea... You could also try the approach one of my teachers used: basically we focused on setting down the basic color/value shapes straight on the canvas, no underpainting, and without worrying about layering or smooth transitions. It's a very simple approach to start with, because you're just laying down chunks of color as you see them and not getting involved in layering or building things up. And it doesn't matter which colors you "start" with, because they all end up side-by-side anyway...
The point of those exercises was to work over the whole image fairly quickly, trying to set down the basic overall colors, values and shapes of the whole image, and trying to see how they relate to each other to make the image. If that makes sense.
Try a bunch of things, see what works. The first thing is to actually slap some paint on the canvas, so you're not staring at a blank canvas all day. Doesn't matter if it's wrong, you'll learn by trying to fix it. And you can always throw it out and start over. You're not working with rare gemstones here... Make a mess, make mistakes, you learn to paint by making lots of bad paintings.