I know that ncurses supports 16-bit colors with the init_pair function and friends. But is it possible to display characters in full RGB color (aka True Color)?
The ncurses FAQ Why only 16 (or 256) colors? goes into some detail, giving the history of this feature, pointing out that the proper term is direct color (since that is based on a standard, while true color is in other places said to be a special case of direct color). Likewise, the xterm FAQ Can I set a color by its number? provides corresponding detail on the history of this feature in xterm.
ncurses 6.1 (January 2018) introduced support for direct color, as illustrated in a recap of the ncurses versus slang history. That includes an example program picsmap which uses the RGB extension (documented in user_caps(5)).
Because the number of colors in 24-bit RGB is larger than the range of numbers supported in the original (signed!) 16-bit numbers (see term(5)), it was necessary to provide for larger numbers. ncurses 6.1 does this with minimal change to existing applications by adding to the opaque TERMINAL structure, and adding functions which can manipulate the extended numbers. It was not necessary to change the ABI (currently 6 since August 2015), because none of the documented features changed their binary interface.
To use the RGB feature in ncurses it is necessary to have the proper terminal description. xterm-direct is used for xterm. This example sets the RGB flag and overrides the color features (but reserves the first 8 ANSI colors, making it a workable hybrid):
xterm+direct|xterm with direct-color indexing,
RGB,
colors#0x1000000, pairs#0x10000, CO#8,
initc@, op=\E[39;49m,
setab=\E[%?%p1%{8}%<%t4%p1%d%e48\:2\:\:%p1%{65536}%/%d\:%p1
%{256}%/%{255}%&%d\:%p1%{255}%&%d%;m,
setaf=\E[%?%p1%{8}%<%t3%p1%d%e38\:2\:\:%p1%{65536}%/%d\:%p1
%{256}%/%{255}%&%d\:%p1%{255}%&%d%;m,
setb@, setf@,
Other terminals have their corresponding flavors. Because the feature is documented, with examples, it is possible for others to customize terminal descriptions as needed.
picsmap is one of the test-programs for ncurses, which is available separately as "ncurses-examples". Tutorials are off-topic; the source-code is readily available.