question

You can see : ColorBar - A Gradient Colored ProgressBar
(VB, but same code in C#)

One way is to derive your own progressbar from the progressbar control, and then use PathGradientBrush or LinearGradientBrush to paint its color in the OnPaint method.

An example using PathGradientBrush:

     public class MyProgressBar : ProgressBar
     {
         public MyProgressBar()
         {
             this.SetStyle(ControlStyles.UserPaint, true);
         }
    
         protected override void OnPaint(PaintEventArgs e)
         {
             Rectangle rec = e.ClipRectangle;
             GraphicsPath path = new GraphicsPath();
             path.AddRectangle(rec);
    
             PathGradientBrush pthGrBrush = new PathGradientBrush(path);
    
             pthGrBrush.CenterColor = Color.FromArgb(255, 0, 0, 255);
    
             Color[] colors = { Color.Red,Color.Yellow};
             pthGrBrush.SurroundColors = colors;
    
             rec.Width = (int)(rec.Width * ((double)Value / Maximum)) - 4;
             if (ProgressBarRenderer.IsSupported)
                 ProgressBarRenderer.DrawHorizontalBar(e.Graphics, e.ClipRectangle);
             rec.Height = rec.Height - 4;
             e.Graphics.FillRectangle(pthGrBrush, 2, 2, rec.Width, rec.Height);
         }
     }

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you might mean this --> https://www.codeproject.com/Articles/31216/NeroBar