The flip operation on colored inner-triangle-free triangulations of a convex polygon is studied. It is shown that the affine Weyl group (C) over tilde (n) acts transitively on these triangulations by colored flips, and that the resulting colored flip graph is closely related to a lower interval in the weak order on (C) over tilde (n). Lattice properties of this order are then applied to compute the diameter. (C) 2009 Elsevier Inc. All rights reserved.4517795