compose_color_palette.kt

private class ColorWheel(diameter: Int) {
  private val radius = diameter / 2f

  private val sweepGradient = SweepGradientShader(
    colors = listOf(
      Color.Red, Color.Magenta, Color.Blue,
      Color.Cyan, Color.Green, Color.Yellow,  Color.Red
    ),
    center = Offset(radius, radius)
  )

  private val radialGradient = RadialGradientShader(
    center = Offset(radius, radius),
    radius = radius,
    colors = listOf(Color.White, Color.Transparent),
  )

  val image = ImageBitmap(diameter, diameter)
    .also { bitmap ->
      val canvas = Canvas(bitmap)
      val center = Offset(radius, radius)
      val paint = Paint().apply { shader = sweepGradient }
      canvas.drawCircle(center, radius, paint)
      val saturation = Paint().apply { shader = radialGradient }
      canvas.drawCircle(center, radius, saturation)
    }
}