Many ancient paintings, in particular frescoes, have some parts ruined by time and events. Sometimes one or more non-negligible regions are completely lost, leaving a blank that is called by restaurateurs a 'lacuna'. The general restoration philosophy adopted in these cases is to paint the interior part of the lacuna with an achromatic or non-neutral uniform color carefully selected in order to minimize its overall perception. In this paper, we present a computational model, based on a well-established variational theory of color perception, that may facilitate the job of a restaurateur by providing both achromatic and non-neutral colors which minimize the local contrast with the surrounding parts of the fresco.