The runtime selects a background intensity. Because your program does not control this, the runtime relies solely on configuration options to do this. If you do not set any options, then the runtime uses a default intensity based on your hardware and operating environment.
You may choose your own intensity by using the BACKGROUND-INTENSITY configuration variable. It can be set to one of the values "0", "1" or "2".
When it's set to "0", the runtime uses the default intensity.
When it's set to "1", the runtime uses low-intensity.
When it's set to "2", the runtime uses high-intensity.
There are two important exceptions:
The first is that the runtime always assigns low-intensity to the background if the background
color is black. Using high-intensity would cause the background to be dark gray, which tends
to make the screen look muddy.
The second exception is that many devices do not support a background
intensity independent from the foreground intensity (most terminals, for example). If
this is the case, the runtime uses a convention of declaring the background
intensity to be low-intensity.
After setting the initial background intensity, the runtime selects the initial foreground intensity. If your program specifies an intensity, then that is the intensity used.
If your program uses the default intensity, then the setting of the configuration option FOREGROUND-INTENSITY determines the intensity:
If it is set to "1", then the runtime uses low-intensity.
Setting it to "2" causes the runtime to use high-intensity.
If it is set to "0", then the runtime uses the default intensity for the output device.
If your program specifies a default intensity, then the runtime will never assign high-intensity if the foreground is black. As with the background, we do this to prevent a washed-out appearance. There's one exception to this rule. The runtime will assign high-intensity to a black foreground if the output device does not support independent background intensities. In this case, the device will typically show the background in high-intensity and keep the foreground black. Note that if your program explicitly sets high-intensity, then that will be used regardless of the foreground color.