size(7cm,0);

path chemin=(0,0){dir(45)}..(1,0)..(1,.5)..(0,1)..cycle;

pen[] p={red,green,blue,magenta};
pair[] points={(0,0),(1,0),(1,1),(0,1)};
int[] edge_flag={0,0,0,2};
draw(chemin);
gouraudshade(chemin,p,points,edge_flag);
dot(chemin);
draw(points[0]--points[1]--points[2]--points[3]--cycle,dashed);