Most people are used to being able to send certain color/formatting codes to a terminal. For instance, most terminal emulators support the ANSI color escape codes. In fact, xterm goes beyond this and has supported 256 color output for about ten years now and most other terminal emulators also support this extension.
The reality is that assuming that a terminal supports ANSI color codes is a bad idea for a bunch of reasons.
The most important reason is that the file descriptor you're printing to might
not be a TTY. This commonly happens when stdout is redirected to another program
or a file. If you emit ANSI escape codes then the output is no longer "plain
text" and it makes a bunch of common operations more difficult. It can break
programs like grep if the search term spans escape codes. It also causes weird
things to happen when you use programs like less (which will escape the escape
codes by default).
Even if the file descriptor you're printing to is a TTY, not all TTYs support ANSI escape codes. For instance, you could be sitting at an actual 1960s teletype machine that doesn't support color output. Or you could be on an ancient terminal that does support color output, but doesn't use the ANSI escape codes.
If you do want to print color at a minimum you should use
isatty(3) to check if the file descriptor
(typically stdout or stderr) is a TTY. Most other languages have native bindings
to this; for instance, in Python you can use
os.isatty and in bash you
can use test -t. If you do this you're not actually guaranteed that the
terminal supports ANSI escape codes, but most modern terminals (or more likely,
terminal emulators) do support them so it's a fairly safe assumption. If you
want to be really pedantic you should actually make a best effort to check the
output capabilities of the TTY device; you can do this with
termcap/terminfo, which typically
would be done for you by a higher level library like
ncurses.
tl;dr If you're going to send ANSI color escape codes to stdout, at the very
minimum please check that this is reasonable with isatty first.