What is the absolute lowest level of "drawing" abstraction in GNU/Linux?

What is the absolute lowest level of "drawing" abstraction in GNU/Linux?

Motivation - Write a program in C (and Assembly, if required) to color a
rectangular area in the screen red.
STRICT requirements - GNU/Linux running with the bare minimum utilities
and interfaces in text/console mode. So no X (or equivalent like
Wayland/Mir), no non-default (outside POSIX, LSB, etc. provided by the
kernel) library or interface and no extra assumptions except the presence
of the device driver for the monitor.
Effectively, what I am looking for is information on how to write a
program which will eventually send a signal through the VGA port and cable
to the monitor to color a particular portion of the screen red.
Apologies if this sounds rude, but no "Why do you want to do this?" or
"Why don't you use ABC library?" answer. I am trying to understand how to
write an implementation of the X server or a kernel framebuffer (/dev/fb0)
library for example. It is ok to provide a link to the source of a C
library.