Henri Matisse: A sea urchin in Matisse's style would be bold and vibrant, with exaggerated colors and simplified forms. The sea urchin might be depicted in bright reds and purples, set against a backdrop of contrasting colors like deep blues and sunny yellows. The composition would be flat and decorative, with playful patterns that evoke the lively essence of the underwater world, capturing the joy and vibrancy of ocean life.