The Collaborative International Dictionary of English v.0.48:

Vandyke \Van*dyke"\ (v[a^]n*d[imac]k"), a.
   Of or pertaining to the style of Vandyke the painter; used or
   represented by Vandyke. "His Vandyke dress." --Macaulay.
   [Written also Vandyck.]
   [1913 Webster]

   Vandyke brown (Paint.), a pigment of a deep semitransparent
      brown color, supposed to be the color used by Vandyke in
      his pictures.

   Vandyke collar or Vandyke cape, a broad collar or cape of
      linen and lace with a deep pointed or scalloped edge, worn
      lying on the shoulders; -- so called from its appearance
      in pictures by Vandyke.

   Vandyke edge, an edge having ornamental triangular points.
      [1913 Webster]

WordNet (r) 3.0 (2006):

Vandyke brown
    n 1: a moderate brown color