(define (hex a r g b)
  (string-append
   (if (< a 16)
       (format "0~x" a)
       (format "~x" a))
   (if (< r 16)
       (format "0~x" r)
       (format "~x" r))
   (if (< g 16)
       (format "0~x" g)
       (format "~x" g))
   (if (< b 16)
       (format "0~x" b)
       (format "~x" b))))