Like page objects, groups and layers, artboards have their own color and opacity properties.
Any color (gradient or solid) can be applied to an artboard using the Color or Swatches panels or the Fill Tool. Colors can be applied to the artboard's stroke or fill.
If a color is applied to the artboard's fill, it will appear behind any objects placed on the artboard—therefore acting as a background color.
If a color is applied to the artboard's stroke, it will appear over any objects placed on the artboard—therefore acting as an overlay.
For information on applying colors to an artboard's fill or stroke, see the Selecting colors and Gradient editing topics.
The artboard's fill and stroke colors can also have a solid opacity or gradient transparency applied. This can be achieved using the Color panel or the Transparency Tool, respectively. For more information, see the Selecting colors and Transparency topics.
However, artboards also have an overall Opacity property which is controlled on the Layers panel. Adjusting the opacity of the artboard in this way will affect all objects placed on the artboard.