Define color with customized name

Question

How to define a specified RGB value as color with customized name in Vim script, much like the existing color Black, Gray, etc.

Answer 1

tl;dr: it seems you can't do this. Sorry :-(

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Longer:

For terminals, there is a static list of 28 colors in syntax.c which defined the names (around line 7306):

static char *(color_names[28]) = {
            "Black", "DarkBlue", "DarkGreen", "DarkCyan",
            "DarkRed", "DarkMagenta", "Brown", "DarkYellow",
            "Gray", "Grey",
            "LightGray", "LightGrey", "DarkGray", "DarkGrey",
            "Blue", "LightBlue", "Green", "LightGreen",
            "Cyan", "LightCyan", "Red", "LightRed", "Magenta",
            "LightMagenta", "Yellow", "LightYellow", "White", "NONE"};

The exact value of those colours will depend on your terminal, but the most used is probably:

/* for xterm with 256 colors... */
static int color_numbers_256[28] = {0, 4, 2, 6,
                                    1, 5, 130, 130,
                                    248, 248,
                                    7, 7, 242, 242,
                                    12, 81, 10, 121,
                                    14, 159, 9, 224, 13,
                                    225, 11, 229, 15, -1};

There is no way I see in the code to add a colour name from VimScript.

For gVim, the code is slightly different. The function gui_mch_get_color() in the gui_*.c files handles this. The precise content differs for every GUI backend,

For example, for the GTK2 GUI it uses the gdk_color_parse() function, for X11 it uses XParseColor, and for Windows it seems to use a static list. Perhaps some of these systems allow defining custom colour names; but this is not something you can do from within VimScript as such.