The context and logic for choosing default --color screen attributes
(foreground and background colors, etc.) are the following.
-- There are eight basic colors available, each with its own
nominal luminosity to the human eye and foreground/background
codes (black [0 %, 30/40], blue [11 %, 34/44], red [30 %, 31/41],
magenta [41 %, 35/45], green [59 %, 32/42], cyan [70 %, 36/46],
yellow [89 %, 33/43], and white [100 %, 37/47]).
-- Sometimes, white as a background is actually implemented using
a shade of light gray, so that a foreground white can be visible
on top of it (but most often not).
-- Sometimes, black as a foreground is actually implemented using
a shade of dark gray, so that it can be visible on top of a
background black (but most often not).
-- Sometimes, more colors are available, as extensions.
-- Other attributes can be selected/deselected (bold [1/22],
underline [4/24], standout/inverse [7/27], blink [5/25], and
invisible/hidden [8/28]). They are sometimes implemented by
using colors instead of what their names imply; e.g., bold is
often achieved by using brighter colors. In practice, only bold
is really available to us, underline sometimes being mapped by
the terminal to some strange color choice, and standout best
being left for use by downstream programs such as less(1).
-- We cannot assume that any of the extensions or special features
are available for the purpose of choosing defaults for everyone.
-- The most prevalent default terminal backgrounds are pure black
and pure white, and are not necessarily the same shades of
those as if they were selected explicitly with SGR sequences.
Some terminals use dark or light pictures as default background,
but those are covered over by an explicit selection of background
color with an SGR sequence; their users will appreciate their
background pictures not be covered like this, if possible.
-- Some uses of colors attributes is to make some output items
more understated (e.g., context lines); this cannot be achieved
by changing the background color.
-- For these reasons, the GCC color defaults should strive not
to change the background color from its default, unless it's
for a short item that should be highlighted, not understated.
-- The GCC foreground color defaults (without an explicitly set
background) should provide enough contrast to be readable on any
terminal with either a black (dark) or white (light) background.
This only leaves red, magenta, green, and cyan (and their bold
counterparts) and possibly bold blue. Default colors. The user can overwrite them using environment
variable GCC_COLORS.
1.8.1.1