Updated 2003-06-28 02:40:28

Suppose you have a color, perhaps the result of a tk_chooseColor operation, or some sort of "$widget configure -fg" introspection and transformation. Is there a way to say that color in English?

Sure. Just use the code below:
    proc get_color_name {color_value permitted_list} {
        set convenient_large_number 10000
        set least_distance $convenient_large_number
        set set_name unknown

        if [regexp #(.*) $color_value -> rgb] {
            scan $rgb %2x%2x%2x r0 g0 b0
        } else {
                # Assume it's a known color name.  In production, one
                #    ought to handle exceptions.
            foreach {r0 g0 b0} [get_rgb $color_value] {}

        foreach name $permitted_list {
            foreach {r g b} [get_rgb $name] {}
                # One can make a case for several other metrics.  This
                #    has the advantages of being mathematically robust
                #    and maintainable from a software standpoint.
            set d [expr abs($r - $r0) + abs($g - $g0) + abs($b - $b0)]
            if {!$d} {
                return $name
            if {$d < $least_distance} {
                # puts "$name, at ($r, $g, $b), is within $d of ($r0, $g, $b0)."
                set least_distance $d
                set best_name $name
        return "$best_name +/ $least_distance"
        # Where are these formats documented?
    proc get_rgb color_name {
            # If it's sufficiently important, one might replace the [winfo ...]
            #    with a table lookup.  At that point, this script becomes "pure Tcl".
        foreach part [winfo rgb . $color_name] {
            scan [format %4x $part] %2x%2x first second
            lappend list $first
        return $list

In practical use, the two arguments each come in a couple of varieties. permitted_list lists recognized color names. It might be a restricted palette, like
    set short_list {red orange yellow green blue violet}

(notice 'indigo' can't appear; Tk doesn't know it). The other most likely useful assignment appears in "Named Colors", where
    set COLORS { snow {ghost white} {white smoke} gainsboro ... gray99}

includes all the colors Tk knows.

The first argument is either a color name, as might appear in $permitted_list, or an RGB value, like "#c71382" or "#9400d3". A slightly more complex procedure might parse other color formats.

As it stands, then, get_color_name computes such values as
    get_color_name yellow $short_list     -> yellow
    get_color_name sienna $short_list     -> red +/- 222
    get_color_name {light coral} $COLORS  -> light coral
    get_color_name #39a051 $COLORS        -> sea green +/ 38