how to use two colors in label for text in c# in Windows Application

public void RenderRainbowText(string Text, PictureBox pb)
{
 pb.Image = new Bitmap(pb.Width, pb.Height);
 using (Graphics g = Graphics.FromImage(pb.Image))
 {
    SolidBrush brush = new SolidBrush(Color.White);
    g.FillRectangle(brush, 0, 0,
    pb.Image.Width, pb.Image.Height);
    string[] chunks = Text.Split(',');
    brush = new SolidBrush(Color.Black);
    SolidBrush[] brushes = new SolidBrush[] {
        new SolidBrush(Color.Red),
        new SolidBrush(Color.Green),
        new SolidBrush(Color.Blue),
        new SolidBrush(Color.Purple) };
    float x = 0;
    for (int i = 0; i < chunks.Length; i++)
    {
     g.DrawString(chunks[i], pb.Font, brushes[i], x, 0);
     x += (g.MeasureString(chunks[i], pb.Font)).Width;
     if (i < (chunks.Length - 1))
     {
        g.DrawString(",", pb.Font, brush, x, 0);
        x += (g.MeasureString(",", pb.Font)).Width;
     }
    }
 }
}