Heat a large pot of water to boiling. Lightly salt the water, add the cauliflower and lemon zest, and cook until the caulflower is crisp-tender, 4 to 5 minutes. Drain. Refresh in cold water and drain again. Transfer to a large mixing bowl.
Heat, but do not boil, the olive oil in a small saucepan. Remove the pan from the heat, drop in the pepper flakes, and let steep until the oil is red, about 15 minutes. Strain the oil, discarding the flakes. You may add a little paprika to heighten the color, if you want. Let the oil cool completely.
Combine the vinegar and garlic in a mixing bowl. Gradually whisk in the red pepper oil, then stir in the capers. Season to taste with salt. Pour the vinaigrette over the cauliflower and toss to coat. Let marinate at room temperature a few hours.