Create an image of a pristine white calla lily in full bloom, elegantly positioned in a tranquil waterside garden at dusk. The calla lily should have smooth, pure white petals curving gracefully around a yellow spadix. The waterside garden should include a serene pond reflecting the twilight sky, surrounded by lush foliage and other blooming flowers. The background features willow trees with their branches gently touching the water’s surface and a quaint wooden footbridge partially visible. Use a serene, pastel art style to convey the calm and peaceful atmosphere of the scene. The angle should be a close-up, showcasing the calla lily's elegant form against the soft, muted colors of the garden at dusk.