How do I layer a bitmap in C# winforms?

<pre>void GenerateChalkStripes()
		{
			//ADD A REAL RANDOM ALGORITHM LATER
			GenerateColorScheme(2);

            /* If i comment this section out like this
			using (Graphics gfx = Graphics.FromImage(bmp))
			using (SolidBrush brush = new SolidBrush(colors[0]))
			{
				gfx.FillRectangle(brush, 0, 0, 400, 400);
			}
            */
/*It works. It generates the stripes as I want it to, but with a background in the color "Control" (the default for WinForms). I want it to draw this background, and then add the stripes over the background... BUT when I try this by removing this comment, it simply does the background and ignores the stripes. Why? How is it not possible to layer pixels? Do I to like print it out to the pictureBox1, then create a new bitmap variable from the pictureBox1 or something? Or have I simply forgotten to like close the brush tool or something stupid?*/

			Bitmap bmp = new Bitmap(400, 400);
			Random r = new Random();
			int numOfStripes = r.Next(4, 16);
			MessageBox.Show(numOfStripes.ToString());
			int stripeThickness = r.Next(1, 16);
			int cutoutThickness = r.Next(1, 8);
			int numOfCutouts = r.Next(20, 80);

			int thicknessPerThickBar = (400 /*- (numOfStripes * stripeThickness)*/) / numOfStripes;

			using (Graphics gfx = Graphics.FromImage(bmp))
			using (SolidBrush brush = new SolidBrush(colors[1]))
			{
				for (int i = 0; i < numOfStripes; i++)
				{
					int startX = thicknessPerThickBar * i;
					gfx.FillRectangle(brush, startX, 0, stripeThickness, 400);
				}
			}

			pictureBox1.Image = bmp;
		}