Melt the butter over low heat in a 3-quart or larger saucepan.
Meanwhile, peel and finely chop the onion.
Add it to the pan and cook until soft, about 4 to 5 minutes.
Stir frequently.
While the onion cooks, pour the white corn kernels into a colander and rinse under lukewarm tap water for 1 minute.
Drain well.
Pour the corn and 1/2 cup of the milk into a food processor work bowl and process on high with the steel blade until pureed, about 15 seconds.
Set aside.
Pour the yellow corn kernels into a colander and rinse under lukewarm tap water for 1 minute.
Drain well.
To the saucepan, add the yellow corn kernels, the white corn mixture and the remaining 1/4 cup milk.
Raise the heat to medium and cook, stirring constantly, until the mixture comes to a boil.
Reduce the heat to low and continue to cook, stirring from time to time.
Add the salt and pepper to taste.
Continue to cook until heated through or until ready to serve.