Like page objects, groups and layers, artboards have their own colour and opacity properties.
Any colour (gradient or solid) can be applied to an artboard using the Colour or Swatches panels or the Fill Tool. Colours can be applied to the artboard's stroke or fill.
If a colour is applied to the artboard's fill, it will appear behind any objects placed on the artboard—therefore acting as a background colour.
If a colour 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 colours to an artboard's fill or stroke, see the Selecting colours and Gradient editing topics.
The artboard's fill and stroke colours can also have a solid opacity or gradient transparency applied. This can be achieved using the Colour panel or the Transparency Tool, respectively. For more information, see the Selecting colours 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.