Aqueous paint composition and method for forming coating

A method for forming a multilayer coating film comprising the steps of:

• (1) applying an aqueous first colored coating composition (X) to a substrate to form a first colored coating film thereon;
• (2) applying an aqueous second colored coating composition (Y) to the uncured first colored coating film to form a second colored coating film thereon;
• (3) applying a clear coating composition (Z) to the uncured second colored coating film to form a clear coating film thereon;
• and
• (4) heating to simultaneously cure the uncured first colored coating film, uncured second colored coating film, and uncured clear coating film,
• the aqueous first colored coating composition (X) comprising an aqueous film-forming resin (A), a crosslinking agent (B), and acrylic urethane resin composite particles (C), the acrylic resin of the acrylic urethane resin composite particles (C) being obtained by reacting, as constituent monomers, 30 to 80 mass % of a polymerizable unsaturated monomer (c-1-1) having one polymerizable unsaturated group per molecule and having a C4-22 alkyl group, 1 to 20 mass % of a polymerizable unsaturated monomer (c-2) having two or more polymerizable unsaturated groups per molecule, and 0 to 69 mass % of a polymerizable unsaturated monomer (c-1-2) having one polymerizable unsaturated group per molecule and being other than the polymerizable unsaturated monomer (c-1-1),
• wherein the urethane resin of the acrylic urethane resin composite particles (C) is obtained from a polyol component, and a polyisocyanate component, and
• wherein the urethane resin component in the acrylic urethane resin composite particles (C) has a weight average molecular weight of 20,000 to 100,000.