[racket] make only selected color to transparent

it's about 10 hours since I stuck at overlaying several images
(make only selected color to transparent)
could somebody give me an little advice?
just little hint about big picture would be very helpful for me.
please~~!

#lang racket/gui
(define base-dc (make-object bitmap-dc% (make-object bitmap% 320 320 #f) ))
(define m-canvas%
  (class canvas%
    (override on-paint)
    (define on-paint (λ () (send (send this get-dc) draw-bitmap (send
base-dc get-bitmap) 0 0)))
    (super-new)))
(define color (make-object color% "red"))
(define f (new frame% (label "a")))
(define c (new m-canvas% (parent f)(min-width 320) (min-height 320)))
(send base-dc draw-bitmap (make-object bitmap% "./src/arena.png") 0 0 )
(send base-dc set-argb-pixels 0 0 288 256 (make-bytes (* 4 288 256)))
(send base-dc draw-bitmap (make-object bitmap% "./src/char4.png") 0 0 )
(send f show #t)
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