Bring a large pot of salted water to a boil. Add potatoes, cooking until just soft. Drain and cool to room temperature.
While the potatoes are cooking, roast the peppers and onion. Brush with olive oil. OPTION 1: grill on medium for 12-14 minutes until charred, turning often. OPTION 2: Preheat oven to 450 degrees. Roast on a sheet pan until charred, 20 minutes. Allow peppers and onion to cool to room temperature and then dice.
Combine potatoes, peppers, onions, and olives large bowl. Drizzle with the Olive & Cilantro Vinaigrette. Season to taste with salt and pepper, garnish with chopped cilantro. Serve slightly warm or at room temperature.
Blend all ingredients in a food processor. Season to taste with salt and pepper.
To prevent a transfer of color you can boil the purple and sweet potatoes separately.
I do not salt the peppers and onions for roasting as the dressing can be rather salty depending on your olives. Salt and pepper at the end, if needed.
The peppers and onions can be made ahead, so I typically grill them when I have the grill on for other things.