Aero: How to draw solid (opaque) colors on glass?

Seems to work OK for me. With the lack of a full code example I’m assuming you’ve got your compositing mode wrong.

public void RenderGdiPlus()
{
    List<string> colors = new List<string>(new string[] { "000000", "ff0000", "00ff00", "0000ff", "ffffff" });
    List<string> alphas = new List<string>(new string[] { "00", "01", "40", "80", "c0", "fe", "ff" });
    Bitmap bmp = new Bitmap(200, 300, System.Drawing.Imaging.PixelFormat.Format32bppArgb);

    Graphics graphics = Graphics.FromImage(bmp);
    graphics.CompositingQuality = System.Drawing.Drawing2D.CompositingQuality.HighQuality;
    graphics.PixelOffsetMode = System.Drawing.Drawing2D.PixelOffsetMode.None;
    graphics.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.None;

    graphics.CompositingMode = System.Drawing.Drawing2D.CompositingMode.SourceCopy;
    graphics.CompositingQuality = System.Drawing.Drawing2D.CompositingQuality.HighQuality;
    SolidBrush backBrush = new SolidBrush(Color.FromArgb(254, 131, 208, 129));
    graphics.FillRectangle(backBrush, 0, 0, 300, 300);

    graphics.CompositingMode = System.Drawing.Drawing2D.CompositingMode.SourceOver;
    Pen pen = new Pen(Color.Gray);
    for (int row = 0; row < alphas.Count; row++)
    {
        string alpha = alphas[row];
        for (int column=0; column<colors.Count; column++)
        {
            string color = "#" + alpha + colors[column];
            SolidBrush brush = new SolidBrush(ColorTranslator.FromHtml(color));
            graphics.DrawRectangle(pen, 40*column, 40*row, 32, 32);
            graphics.FillRectangle(brush, 1+40*column, 1+40*row, 31, 31);
        }
    }

    Graphics gr2 = Graphics.FromHwnd(this.Handle);
    gr2.CompositingMode = System.Drawing.Drawing2D.CompositingMode.SourceCopy;
    gr2.CompositingQuality = System.Drawing.Drawing2D.CompositingQuality.HighQuality;
    gr2.PixelOffsetMode = System.Drawing.Drawing2D.PixelOffsetMode.None;
    gr2.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.None;
    gr2.DrawImage(bmp, 0, 0);
}