make prompt colors configurable via GOSH_COLORS

- GOSH_COLORS='user=..:root=..:path=..' with ANSI SGR codes,
  empty value disables the color for that part
- only digits and ';' are accepted, invalid entries are ignored
- read before every prompt, so 'set GOSH_COLORS=...' takes effect immediately
- document in help and README, including example color schemes

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

main.go

@@ -309,31 +309,82 @@ func getPrompt() string {
 		wd = "~" + strings.TrimPrefix(wd, home)
 	}
 	
-	return buildPrompt(username, hostname, wd, os.Geteuid() == 0, useColor())
+	var colors *promptColors
+	if useColor() {
+		c := parseColors(os.Getenv("GOSH_COLORS"))
+		colors = &c
+	}
+	return buildPrompt(username, hostname, wd, os.Geteuid() == 0, colors)
 }
 
-// ANSI escape sequences used for the prompt
-const (
-	colorReset = "\x1b[0m"
-	colorUser  = "\x1b[1;32m" // bold green
-	colorRoot  = "\x1b[1;31m" // bold red
-	colorPath  = "\x1b[1;34m" // bold blue
-)
+// promptColors holds the SGR parameters (e.g. "1;32") for the parts of the
+// prompt. An empty value means no color for that part.
+type promptColors struct {
+	user string // user@host
+	root string // user@host when running as root
+	path string // working directory
+}
+
+// defaultColors: bold green user, bold red root, bold blue directory
+var defaultColors = promptColors{user: "1;32", root: "1;31", path: "1;34"}
+
+// parseColors reads a GOSH_COLORS value like "user=1;33:root=1;31:path=36"
+// and overrides the corresponding defaults. Unknown keys and values that are
+// not plain SGR parameters (digits and ';') are ignored, so the variable
+// cannot be used to write arbitrary escape sequences to the terminal.
+func parseColors(spec string) promptColors {
+	colors := defaultColors
+	for _, entry := range strings.Split(spec, ":") {
+		key, value, ok := strings.Cut(entry, "=")
+		if !ok || !isSGR(value) {
+			continue
+		}
+		switch key {
+		case "user":
+			colors.user = value
+		case "root":
+			colors.root = value
+		case "path":
+			colors.path = value
+		}
+	}
+	return colors
+}
+
+// isSGR reports whether s consists only of digits and ';'. The empty string
+// is allowed and disables the color.
+func isSGR(s string) bool {
+	for _, r := range s {
+		if (r < '0' || r > '9') && r != ';' {
+			return false
+		}
+	}
+	return true
+}
+
+// colorize wraps text in the SGR sequence for sgr, or returns it unchanged if
+// sgr is empty.
+func colorize(text, sgr string) string {
+	if sgr == "" {
+		return text
+	}
+	return "\x1b[" + sgr + "m" + text + "\x1b[0m"
+}
 
 // buildPrompt formats the prompt as user@host:path $. The parts are sanitized
 // before the color codes are added, so only our own escape sequences end up
-// in the prompt.
-func buildPrompt(username, hostname, path string, root, color bool) string {
+// in the prompt. colors == nil disables colors.
+func buildPrompt(username, hostname, path string, root bool, colors *promptColors) string {
 	userHost := sanitize(username + "@" + hostname)
 	path = sanitize(path)
-	if !color {
+	if colors == nil {
 		return userHost + ":" + path + " $ "
 	}
-	userColor := colorUser
+	userColor := colors.user
 	if root {
-		userColor = colorRoot
+		userColor = colors.root
 	}
-	return userColor + userHost + colorReset + ":" + colorPath + path + colorReset + " $ "
+	return colorize(userHost, userColor) + ":" + colorize(path, colors.path) + " $ "
 }
 
 // useColor reports whether the prompt should be colored: stdout must be a
@@ -698,6 +749,8 @@ Features:
   - Tab completion for commands and file paths
   - Tilde expansion (~, ~/path) at the start of unquoted words
   - Colored prompt (user@host green, red for root; directory blue),
+    configurable with $GOSH_COLORS, e.g. GOSH_COLORS='user=1;33:path=36'
+    (keys user, root, path; values are ANSI SGR codes, empty = no color);
     disabled with $NO_COLOR, TERM=dumb or when output is not a terminal
   - Environment variable expansion ($VAR, ${VAR}, ${VAR:-default});
     using an unset variable is an error, use ${VAR:-} for an empty default`)