Combine oats, sugar and butter, mixing until crumbly. Place in a shallow baking pan. Heat in preheated moderate oven (350 F.) about 10 minutes, stirring occasionally. Firmly press onto bottom of a 7- half inch spring form pan. Chill. Using ice cream dipper, scoop sherbet balls onto cooky sheet; freeze until firm. Alternating colors, firmly press sherbet balls into crumb-lined pan. Freeze until firm. To unmold, loosen sides with spatula. Invert onto plate; cut in slices. NOTE: If spring form pan isn't available, firmly press crumb mixture onto bottom and part-way up the sides of a 2 litre mixing bowl.