By default a control's background color will be the same as the container's background color.

You can set the control's BackColor property to Transparent in the Designer. Select the control in the Designer, select the BackColor property in the Properties pane and then select the drop-down arrow that appears in the property value. On the Web tab, select Transparent.

For a translucent (semi-opaque) background, in the BackColor property in the Properties pane you can type "128, 0, 255, 0" for a 50% opacity green background with 50% opacity. The alpha value ranges from 0 (transparent) to 255 (opaque).

Or you can programmatically set the alpha value of the control's background.

using System.Drawing;

button1.BackColor = Color.Transparent;
button2.BackColor = Color.FromArgb( 64, 0, 255, 0 ); 

In this example, button1 will have a transparent background, and button2 will have a 25% opacity green background.

Note that a form's BackgroundImage property, if set, will obscure the form's BackColor property. That makes it non-obvious why a control placed on the form has the background color it does. For example: create a new form; set the form's BackColor to Red; set the form's BackgroundImage to .\My Pictures\Sample Pictures\Winter.jpg; now drag a Button onto the form. The button's background is displayed as red in the Designer and at runtime.

George Shepherd, Syncfusion, and Stuart Celarier, Fern Creek, 20 December 2004