Quote CGA Standard Mode:

By setting the high-intensity bit, brighter versions of these modes can be accessed. The 1:1.2 pixel aspect ratio needs to be taken into account when drawing large geometrical shapes on the screen.

It only gives you a dull and bright version of the 4 colours but would allow for basic shading without as much dithering.

Source https://en.wikipedia.org/wiki/... Under Standard Graphics Mode.

So can we use high and low intensity colours for a full 8 colours (4 x 2 shades)?

In this case for this am you can use only 4 colors at atime, High or Low intensity, but only 4, not 8 or 16 with the original tricks sorry.