Richard, this is wonderful! Just the kind of information we were needing. As I was suspecting, "optical red" is one of a whole spectrum of light halos. The color of the halo, if I am digesting this correctly, is determined by the color of the object off of which light is refracting, and the color of what is behind it.
Does the color of the refracted light lean towards the red end of the spectrum in comparison with the color of the object itself in direct light? I have noticed that the atmospheric color, which shows up as an object turns away from the source of light, is usually slightly more towards the red end of the light spectrum than the color of the object in direct light.
What I see if I look closely at his silhouette, is that the background is lighter where his head touches, and that the hair has a slightly redder tinge in places as it touches the background.
If I'm not mistaken, it says that the color of the light source extends itself into surrounding space. This is actually a color influence rather than a repeated color. The color of light blends into the color of the background.
So, if the color of your light was warm, and the color of the head/flesh was was warm, you'd get a "warm halo," so to speak -- wouldn't you?
However, if the color of light is blending with the background, and the light source is warm, and the background is cool, you'd get the cancelling effect of mixing a warm and cool. Again, a question mark.
Let's get some really good minds going on this . . . . . . .
I was thinking of optical red: it is redder than the skintones. But maybe that is simply a darkening effect, i.e., the mixing of the light refracting off the lit surface of skin combined with the color of the background. Certainly the halo of white is like a darkened white, not tending towards yellow.
Julie, in that portrait by Kramskoy, it's the opposite effect: a halo (or inverse circle) effect of the light background intruding into the edge of the darker figure! How clever of you to notice the relationship between this and what we have been talking about! Wow, this is getting exciting. I think the two phenomena are definitely two sides of the same coin (I'm not sure how coins come into this, but anyway...).
Trying to discover the truth of light, or color, I would recommend to ask Garth Herrick why he changed the frame and painted it in a color more alike the colors in his "Apotheoun" !
I think we are discussing different subjects in the same thread.
One subject is the observation of colour in real life, while the second is how we are able through our technique to intensifie or create a colour effect.
Painting a halo that is or is not there is a one of the many options we have that serve a purpose. For example, it is useful to unify the painting, it can help placing a subject IN a certain area by stressing its presence there, it can relate the subject to the background.
A red halo coming from a subject that is not red will alter the temperature of the subject's colour, perhaps turning it slightly toward coolness. I think instead that Sargent's reds are there to deepen the shadows through making them very hot.
Euan Uglow's painting (can't post them, I am referring to his nudes) are dotted with little red reference marks, and often red or orange outlines, that structure the drawing and add importance to the surface the painting, they also affect strongly the colour scheme.
I gave up cadmium red at the time of the famous thread, but found myself in the need of putting it back on the palette to have its power available!
Ilaria
Hybrid Mode
