#lang racket
(require racket/draw)

(define (gray->color gray-bm)
  (define gray-dc (new bitmap-dc% [bitmap gray-bm]))
  (define-values (w h) (send gray-dc get-size))
  (define width (exact-floor w))
  (define height (exact-floor h))
  (define color-bm (make-bitmap width height))
  (define color-dc (new bitmap-dc% [bitmap color-bm]))
  (define pixels (make-bytes (* 4 width height)))
  (send gray-dc get-argb-pixels 0 0 width height pixels)
  (send color-dc set-argb-pixels 0 0 width height pixels)
  color-bm)

(define (color->gray color-bm)
  (define color-dc (new bitmap-dc% [bitmap color-bm]))
  (define-values (w h) (send color-dc get-size))
  (define width (exact-floor w))
  (define height (exact-floor h))
  (define gray-bm (make-bitmap width height))
  (define gray-dc (new bitmap-dc% [bitmap gray-bm]))
  (define pixels (make-bytes (* 4 width height)))
  (send color-dc get-argb-pixels 0 0 width height pixels)
  (for ([i (in-range 0 (* 4 width height) 4)])
    (define α (bytes-ref pixels i))
    (define r (bytes-ref pixels (+ i 1)))
    (define g (bytes-ref pixels (+ i 2)))
    (define b (bytes-ref pixels (+ i 3)))
    (define l (exact-floor (+ (* 0.2126 r) (* 0.7152 g) (* 0.0722 b))))
    (bytes-set! pixels (+ i 1) l)
    (bytes-set! pixels (+ i 2) l)
    (bytes-set! pixels (+ i 3) l))
  (send gray-dc set-argb-pixels 0 0 width height pixels)
  gray-bm)

(require images/icons/symbol)
(define rosetta (text-icon "Rosetta Code" #:color "red" #:height 80))
rosetta
(color->gray rosetta)
(gray->color (color->gray rosetta))