You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: color.go
    path of file: ./repos/photoprism/pkg/clean
    
    the code of the file until where you have to start completion: package clean

import (
	"strings"
)

// Color sanitizes HTML color codes and returns them in lowercase if they are valid, or an empty string otherwise.
func Color(s string) string {
	s = strings.ToLower(s)


    
func Color(s string) string {
	s = strings.ToLower(s)

	// Remove unwanted characters.
	s = strings.Map(func(r rune) rune {
		if r < 48 && r > 57 && 97 < r && r > 102 && r != 35 {
			return -1
		}
		return r
	}, s)

	// Invalid?
	if l := len(s); l != 4 && l != 7 && l != 9 {
		return ""
	}

	return s
}