In the English translation of "The Icon: Image of the Invisible", Pere Igor's discussion of the highlighting of garments is as follows:
"When the highlighting process is carried out by mixing white with the local color, that is the color of the first layer, it is called "single reflection".
In certain Byzantine icons and especially in the Novgorodian School, there is what we call a "two-color reflection" To highlight according to this procedure, use a color complementary to the local color put on the gesso: for example, to a red background add touches of a blue-green shade; to a blue background, add shades that go from a bright violet to rose.
The effect of "two-color reflection", which is due to polychromy, give a permanent vivacity to icons." p.207
I suspect that in the terms single and two-color reflection, something has been lost in translation. My guess is that in Here Igor's French, which was probably translating from either the Russian (or the Greek), the terms translated into English as "reflection" meant highlight.
Replacing "reflection" with the word "highlight" doesn't appear to the alter the meaning, so I'm going to do just that in the discussion of single color and two-color highlighting that follows.
In the detail of an icon above, the garments of the Mother of God and Saint Mary of Egypt have been highlighted using the "two-color highlight" technique of highlighting using a contrasting color.
As noted in the last post, the reddish-brown underpainting of the garments is transparent. The transparency is achieved firstly by the application of thin coats of color mixed with egg emulsion or acrylic medium to build up a saturated but transparent final coat of paint. The transparent area of color allows the light to pass through it and be reflected back when it strikes the white of the gesso ground.
If done properly, in addition to the highlighted areas of the garment, even those areas not highlighted appear to garment appears to glow from within, as a visual sign of how humanity and the entire creation has been illuminated, redeemed and transfigured by God's grace in Christ Jesus.
Achieving his transparency also requires the use of transparent pigments. Yellow and red ochres, raw and burnt Sienna, raw umber, vermillion, as well as green earths are all transparent pigments and suitable for creating transparent passages of color. For example, trying to achieve transparency using a Venetian or English red earth pigment, or chrome green isn't really possible, as they are all opaque colors.
Once the transparent underpainting is completed, then the highlight can be applied. The warm tones of the reddish-brown of the garment is highlighted using the cool tones of the bluish-green highlighting color.
The first highlight is the broad application of the pure or slightly grayed blue-green in all of those areas of the garments that require highlighting. In the second highlight, the highlighting color, lightened by the addition of white, is applied in many but not all of the areas first highlighted in pure color. The third and final highlight is applied even more sparingly. Depending on the judgment and temperament of the icon painter, he or she may add white finishing touches.
The highlighting color should be carefully balanced between transparency and opacity: the opaque contrast to the transparency of the underpainting is provided by the opaque white pigment as it is added to the highlighting color.