By using the unicode character \u2580: ▀ and setting the background and the foreground colours, I can print two pixels in one space instead of one pixel in two spaces. This will double the resolution by printing twice as possible pixels given a certain width.

# use ^[[38;5;$colorindexm to change the foreground
# use ^[48;5;$colorindexm to change the background
printf '\033[38;5;1;48;5;2m\u2580\n'