Why my vim doesn't support hex color code?

Question

When I use the following command to set the color of my vim,

:highlight Error ctermfg=#00afff

it prompts E421: Color name or number not recognized: ctermfg=#00afff

Answer 1

ctermfg and ctermbg use indices into the limited color palettes (8 / 16 / 88 or 256 colors) of terminals and do not support the full 24-bit RGB values.

I think you're looking for the 'termguicolors' option, which makes Vim apply the GUI color attributes to capable terminals. Colors (by name or #RGB) are specified via guifg / guibg; only cterm is used for text attributes.

Answer 2

It's because :h highlight-ctermfg says:

ctermbg={color-nr}              *highlight-ctermbg*
    The {color-nr} argument is a color number.  Its range is zero to
    (not including) the number of tui-colors available.
    The actual color with this number depends on the type of terminal
    and its settings.

[...]

The following (case-insensitive) names are recognized:

                        *cterm-colors*
    NR-16   NR-8    COLOR NAME ~
    0       0       Black
    1       4       DarkBlue
    2       2       DarkGreen
    3       6       DarkCyan
    4       1       DarkRed
    5       5       DarkMagenta
    6       3       Brown, DarkYellow
    7       7       LightGray, LightGrey, Gray, Grey
    8       0*      DarkGray, DarkGrey
    9       4*      Blue, LightBlue
    10      2*      Green, LightGreen
    11      6*      Cyan, LightCyan
    12      1*      Red, LightRed
    13      5*      Magenta, LightMagenta
    14      3*      Yellow, LightYellow
    15      7*      White

The command is not supposed to support hex color mode.