Highlight and contouring for darker skintones tends to be a bit tricky compared to lighter skintones so it's important to perfect the steps.

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Casting light and shadow perfectly (essentially what highlighting and contouring does) on the face requires practice to nail it perfectly over time.

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Contouring is supposed to cast 'shadow' of parts of your face (and vice versa with highlighting). Learn how to get a flawless highlight and contour for dark skin tones in easy steps from the tutorial by Kluermoi above.