Here's a response from an artist that may be of help to you:

The use of white outlines in art : I wish your friend could give a little more info, as to period and type of art, as white outlines mean that the artist had to use a white drawing tool on a dark background, and those instances are almost non-existent. But there a 2 instances in which lines of white can be discerned:
" pentimento", a painting beneath a painting, resulting in white ares left unpainted in the primary level painting coming through the overlying painting
"synopie", an undersketch for a fresco in which the predominant red-ochre color of the drawing is highlighted in white.

Neither of the above represents what I would call a true outline, however. If this mystery word comes to light, let me know, as I have degrees in art history as well as studio art, and frankly have never heard of a word for white outlines.

Edit: And then she added:

The word "chiaroscura" is a general art term meaning from light to dark, and drawings about which that term can be employed often employ white lines and/or highlights .

Last edited by Tromboniator; 01/22/2015 9:21 PM.