Wash completely, then Microwave the potatoes by type on the "1 bake potato" cycle. Peel the sweet potato and cut into bite sized pieces then microwave on the "1 potato" setting(this may take more than 1 time through for each type of potato).
Cool all of the potatoes before you cut them into bite sized pieces(sweet potato already cut).
If your eyes water when you peel an onion, make the horseradish in a well ventilated area.(I reccomend an open area anyway) Peel the horseradish root, then chop into managable pieces, put into food processor and grind. After @2minutes add a 50-50 mixture of water and 5% white vinager in the food processor to stop the enzymatic process to stop the heat. The longer you wait, and the finer the grind, THE HOTTER IT GETS!
In a non-reactive bowl mix the yogurt, mayonnaise, horseradish, cider vinegar, mustard, salt & pepper. Cover and Refrigerate.
After the potatoes cool, cut them into bite sized pieces, cut into bite sized pieces the onions and bell peppers then add to the potatoes.
Fold in the yogurt mixture, and refrigerate the completed salad for at least 2 hours; up to 24 hours.