A method is disclosed that includes the operation below. Vertices in a conflict graph are sorted into a first clique and a second clique, in which the conflict graph corresponds to a layout of a circuit. A first vertex of the vertices is merged with a second vertex of the vertices, to generate a reduced graph, in which the first clique excludes the second vertex, and the second clique excludes the first vertex. A first color pattern of a plurality of color patterns is assigned to a first pattern, corresponding to the first vertex, and a second pattern, corresponding to the second vertex, in the layout according to the reduced graph.