In four separate bowls, dissolve each package of gelatin in 1 cup boiling water. Add 1/2 cup cold water to each and stir. Pour one bowl of blue gelatin into an oiled 10-in. fluted tub pan; chill until almost set, about 30 minutes.
Set other three bowls of gelatin aside at room temperature. Soften unflavored gelatin in remaining cold water; let stand 5 minutes.
Heat milk in a saucepan over medium heat just below boiling. Stir in softened gelatin and sugar until sugar is dissolved. Remove from heat; stir in sour cream and vanilla until smooth. When blue gelatin in pan in almost set, carefully spoon 1-1/2 cups sour cream mixture over it. Chill until almost set, about 30 minutes.
Carefully spoon one bowl of strawberry gelatin over cream layer. Chill until almost set. Carefully spoon 1-1/2 cups cream mixture over the strawberry layer. Chill until almost set. Repeat, adding layers of blue gelatin, cream mixture and strawberry gelatin, chilling in between each. Chill several hours or overnight.
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