The Practical Painter: Class Notes

The first viewing of a painting after a break is gold. Not taking advantage of it is like someone handing you a $20 and you flush it down the toilet. The longer the break, the larger the bill you are flushing away.
Difference between "light" and "highlight" modeling factors

Light - generally facing the light source, does not move on object's surface.
Highlight - functions as a reflection and thus follows "angle of incidence = angle of reflection" from way back in physics class (like a mirror). Highlights will move around on the form as you move your eye position, whereas the light modeling factor will not.

When a highlight is very distinct (glossy surface), the light & halftone modeling factors will be slightly darker & slightly more chromatic- though it's easy to overstate how much the color darkens due to simultaneous contrast.
As the highlight gets less distinct (matte surface), the light & halftone modeling factors will get slightly lighter, & the chroma will get slightly lower. The hue in the light area will shift towards the color of the light more than the halftone. In other words, the halftone remains closer to the local hue of the object.
Bonus highlight fact: On metallic objects, the highlight will take on the hue of the object's local color, and will remain much higher chroma than a non-metallic object. On non-metallic objects, the highlight will be the color of the light mixed with some amount of the local color of the object. The glossier the object, the less of the local color to add.

To make an object look wet, give it a strong highlight, lower the value in the light and halftone, and raise the chroma in the light and halftone. In other words- make it glossy!
Wetting an object (in real life) is a useful trick to temporarily clarify an object's local color.
Bonus Glossy/Matte fact: On a matte surface, the highlight disperses across the surface and lightens its value. This is why glossy paints can achieve darker darks than matte paints. It's also why glossy printer paper and glossy varnishes give a wider range of values.

They're usually physical mistakes, and the physical stuff is what you can't learn from books.
Don't beat yourself up- see if something cool happened. If you're sight-reading on the piano or guitar, if you make a mistake you might discover a nice jazz chord you can use later.
Watch for happy accidents, but don't be a lazy bum and leave them in if they look good by themselves but detract from your painting. However, the physical process by which a mistake happened could be the perfect solution for a problem at another time.

Don't start smearing that color all over the canvas until you've touched a small bit on the canvas to see if it's right.

Two paints will mix generally in straight lines through color space. However, in reality they tend to mix along curved lines in color space.

Mixing two paints together will always produce a mixture that is darker than the lighter of the two paints. How much the value lowers depends on the exact pigments used. In some cases, the mixture can be darker than both paints.
Mixing two paints together will always produce a mixture that is lower in chroma than the more chromatic of the two paints. How much the chroma lowers depends on the exact pigments used.

The further apart the hue of the two initial paints, the lower the resulting chroma.

The perceived color of a paint does not dictate how a paint will mix. Two paints that have the exact same hue, value, and chroma but are made up of different pigments might mix in different ways with a third color.

Thus, it's impossible to say that any abstract color (red, blue, etc) has definite mixing complement when dealing with paint. It is, however, possible for a physical pigment to have a mixing complement that will mix to a neutral gray. Interestingly, a pigment can have more than one mixing complement, and these mixing complements may be different colors.

Sunlight (parallel rays)- terminator edge will be sharp and will "bisect" the object.
Spotlight - the terminator edge will be slightly less sharp as compared to sunlight (but still sharp), and will be slightly closer to the light source, depending on how close the object is to the light.
Diffuse light (north light, cloudy days)- The terminator edge will soften, and move away from the light. The larger the light source (i.e. a window or the sky), the softer the edge of the terminator and the further away from the light.

While the exact line between halftone and light is somewhat arbitrary there's a tendency to make the division between halftone and light about halfway "around the form", where the form would be turned about 45 degrees away the light source (this is known as its "inclination to the light").
In fact, the falloff of light happens much more slowly. Check out this chart from the excellent site huevaluechroma.com. Notice that when the inclination of the surface is 50 degrees to the light, the brightness is still at 82%!
There are two caveats here, though. The first is that brightness is not exactly the same as value in paint, and that this chart assumes a glossy object and ignores the highlight. The percentages would be different for a matte object. Nonetheless, the important point is that the value drops in a way that is much different than we think.

The Practical Painter