Complementary shading
The practice, Byzantine in origin, of rendering shading in the modelling of a figure or drapery in a complementary (that is, contrasting) color, rather than with a darker shade of the same color or with black.
The practice, Byzantine in origin, of rendering shading in the modelling of a figure or drapery in a complementary (that is, contrasting) color, rather than with a darker shade of the same color or with black. The technique often produces an image of greater naturalistic as well as decorative effect.
Michelle Brown. Understanding Illuminated Manuscripts (Malibu, CA: J. Paul Getty Museum in association with the British Library, c1994).