Add the ingredients to a large bowl and mix until just combined.
Pipe into butter and floured donut pans
Bake about 10 minutes at 350F, allow to cool on a wire rack.
For the Glaze
Mix milk into powdered sugar.
Add about 2 tbs of the mixture to several bowls and tint with various intensities of the “colors of the rainbow."
Dip the donuts — carefully — into the mixture to achieve the desired, rainbow color. Repeat.
Spoon a thin layer of each color glaze on a plate and dip a donut or two and then reapply more as needed. This will help keep some nice clean lines instead of a muddy mess.
Measure your flour correctly! Adding too much flour to the recipe is the most common mistake. The best, and easiest way to measure flour is by using a scale. If you don't have one then fluff your flour with a spoon, sprinkle it into your measuring cup, and use a knife to level it off.
You can substitute whole milk yogurt for the sour cream if desired, you won really tell the difference.
When you dip your donuts rotate rotate them around, but don't twist, just a bit to ensure the top is completely glazed. When you pull the donuts up tilt them a it so the stripes are vertical, this will help keep the colors distinct.