Remove the tops and scrub the orange and rainbow carrots with a vegetable brush. Cut the carrots in long diagonal slices. (You want fairly uniform sticks, about 4 inches long by 1/2 inch wide.)
Place the carrots on a sheet pan, drizzle with 3 tablespoons olive oil and sprinkle with 1 teaspoon kosher salt and 1/2 teaspoon pepper. Toss well with your hands and spread out in one layer.
Roast for 15 to 20 minutes, turning once, until the carrots are lightly browned and tender. Sprinkle with the orange zest, orange juice and fleur de sel and toss well. Serve hot or at room temperature.
When I use citrus juice in a recipe, I often add the zest as well for even more flavor.