To readily obtain a synthetic resin molding product having a grain pattern or a marble pattern by a molding machine of general purpose without using a specific molding machine.

A high-density polyethylene having ≤5 melt index (MI) in an amount of 40-60 pts.wt. is compounded with 10-60 pts.wt. of inorganic powder and a proper amount of a colorant, heated, kneaded and pelletized to give a colorant composition for developing a pattern. A proper amount of the composition is compounded into an olefin-based resin, which is molded to give a molding product having a grain pattern or a marble pattern.