A Simple Painter's Walk Across the Arm (notes from a workshop held last week):

It is widely acknowledged that the hardest thing to paint is the human figure, partly because of the obvious fact that it is impossible to fudge it if you do it wrong -- everyone knows intuitively what they could not themselves draw -- but just the complexities of simple skin and shadow are daunting, so much is happening at once.

Example: hold your arm out in bright sunlight or in strong lamplight so that a shadow cuts it in half lengthways. Here is one version of what a painter has to contend with, just walking across an arm (forget about materials, brushes, ability for a moment).

Start at the brightest point, the highlight (if any). In a photograph or a painting this will often give off a glare, which bleaches out the local colour. Local colour is the term for whatever the underlying material is coloured (if it is coloured) in ordinary light. Depending on the strength of the light, skin may give off such a reflected glare (that is why people dust themselves with powder on stage and off). Anyway: we are going to stroll from that highlight across the arm into the shadow. Keep in mind that the colour spectrum goes from hot yellow through cooler redpurple to cool blue.

Just at the edge of the highlight (if there is one) the original colour of the originating light will appear (blue tinge for sky light, yellow for direct sun, orange for late day sun, etc.). Next to that -- moving towards the shadow -- at least to our eyes -- will be a very faint complementary -- yellowish complementing blue, purplish complementing yellow -- then this will be submerged with the arrival of the hot brightest version of the local skin colour. Assume a white North American summer light tan allover. This means that the local skin colour will be a complex mix of yellow, red, and blue -- a pale burnt sienna, except intense because of the bright sun or lamplight. The yellow (let us assume we are in bright sun) part of the colour will prevail in the brown , because it is the warmest colour.

Now as the arm begins to turn away from the light, the arm colour begins to cool down, and the brown shifts from yellowish brown to reddish brown (as mentioned, red is cooler on the spectrum than yellow). Simultaneously, the colour begins to lose its intensity, and starts to go slightly gray: a bit of blue. As we move towards the shadow, the colour goes further from red into the slightest hint of purple, and at the termination zone, there is a moment where the light loses coherence, and then there is a full shadow where the originating light is completely blocked off by the form of the arm as it turns away. Just before that happens, as the arm turns away, the shadows lengthen and illuminate the bumps and crevasses in the skin (just like on the moon, right by the shadow). That is where you can see the texture of the arm. The light (since light rays go straight) is actually crossing over the arm, not going around the arm -- so the termination line must waver accordingly, it is not a simple straight line following the form like a cylinder. The light shatters and splinters into darkness.

Some people argue that just before the plunge into total darkness, there is a last moment of complementary blue even green, coolness again, just because of the effect of the warm light beside it coming to an end (green complementing the red that is now dominating in the warm colour mix of the brown). This is happening in fractions of an inch. When the full shadow hits, there is blackness, and this is the most intense moment of what is called form shadow darkness. After that, as we keep moving, reflected light from our surroundings (which is coloured by a whole range of bounced lights coming off of objects, the sky, etc.) begins to infiltrate the shadow again. As we move across the shadow, the shadow thus gets lighter, the edges fuzzier, and if the main light is cool, the depths of the shadow usually go warm, and vice versa; complementary colours dye the shadow. There may even be reflections of blue from the sky (as happens on white snow in the winter). The local colour reasserts itself under the shadow, but at much less intensity, and often there is a tinge of graygreen in it. The forms under shadow are less defined, and must be painted less intensely so as not to attract undue attention.

If the arm is bent away from the sunlight, then this changes again (the intensities drop further, the light cools even more). If the arm is near the rest of the body, the reflected light in the shadows will be warmer. If it is close to a coloured cloth, it will take on that colour, and so on.

All of this, and some knowledge of the anatomy of the arm would be helpful too....