using var img = new Image<Rgba32>(200, 200);
var brush = new SixLabors.ImageSharp.Processing.PathGradientBrush(
    new[]
    {
        new SixLabors.Shapes.LinearLineSegment(new PointF(0, 0), new PointF(200, 0)),
        new SixLabors.Shapes.LinearLineSegment(new PointF(200, 0), new PointF(200, 200)),
        new SixLabors.Shapes.LinearLineSegment(new PointF(200, 200), new PointF(0, 200)),
        new SixLabors.Shapes.LinearLineSegment(new PointF(0, 200), new PointF(0, 0))
    },
    new[]
    {
        Color.Red,
        Color.Yellow,
        Color.Green,
        Color.DarkCyan
    }
);
img.Mutate(m => m.Fill(brush));
img.Save("out.png");