Put the association ELT into the alist of terminal colors for FRAME.
ELT should be of the form (NAME INDEX R G B) (see `tty-color-alist'
for details).
If the association for NAME already exists in the color alist, it is
modified to specify (INDEX R G B) as its cdr. Otherwise, ELT is
appended to the end of the color alist.
If FRAME is unspecified or nil, it defaults to the selected frame.
Value is the modified color alist for FRAME.