Preheat oven to 350 degrees and spray a donut pan with nonstick spray.
In a large bowl, whisk eggs then slowly add melted butter and whisk until slightly thickened. Add in buttermilk and yogurt and stir until mixture is smooth. Add dry mix and whisk until smooth.
Fill each donut mold 3/4 full with batter, then bake 12-15 minutes until golden. Remove from donut pan and cool completely on a wire rack.
To make the glaze, combine the powdered sugar and milk in a small bowl and whisk until smooth. Divide glaze evenly into six bowls and color each a color of the rainbow.
On a large plate, spoon two tablespoons of each glaze color next to each other, in the order of the rainbow. Gently place each donut into the glaze and slightly twist to completely cover the top of each donut.