In a saucepan, heat milk and sugar over low heat until sugar is dissolved. Soften unflavored gelatin in water; stir into the milk mixture until dissolved. Remove from heat; add vanilla. Cool to lukewarm; blend in sour cream. Pour into a 13-in. x 9-in. dish. Chill until set.
In a saucepan, bring the soda to a boil. Place blue gelatin in bowl; stir in 2-1/2 cups of soda until gelatin is dissolved. Pour into another 13-in. x 9-in. dish. Repeat with raspberry gelatin and remaining soda. Refrigerate until set. Using a 1-1/2-in. round cookie cutter, cut white, blue and red gelatin into rounds. Stack or scatter on a serving plate.