6 hexadecimal digit colours in limited-colour terminals

I have seen that true colour palette uses the 6 digit hexadecimal code
#RRGGBB for colour specification.  Thusly the 6 digit code is incompatible
for 8, 16, and 88 colour terminals. =20

How is it then that colour specification for 16-colour
terminals still use hex values?  Aren't they supposed to use=20
colour names instead?=20

    (((class color) (min-colors 16) (background dark))
         :foreground #ff5ese")
     (((class color) (min-colors 16) (background light))
         :foreground "#ff5ecd")

I have also seen that the hex representation for different
terminal implementations are different.  Meaning that red
is not the same red in another 16 colour terminal emulator.