CAD tip # 10071:

You can use the following LISP utility to recolor layers, changing their color method "palette" (ACI) to Truecolor:

; Layer ACI -> truecolor
(defun C:LAY2TC ( / lay truecol aci R G B)
 (vl-load-com)
 (vlax-for lay (vla-get-layers (vla-get-activedocument (vlax-get-acad-object)))
   (setq truecol (vla-get-truecolor lay))
   (if (= (vla-get-ColorMethod truecol) acColorMethodByACI)
    (progn
      (setq aci (vla-get-ColorIndex truecol))            
      (vla-put-ColorMethod truecol acColorMethodByRGB)
      (vla-put-ColorIndex truecol aci)
      (setq R (vla-get-red truecol))
      (setq G (vla-get-green truecol))
      (setq B (vla-get-blue truecol))
      (vla-setRGB truecol R G B)
      (vla-put-truecolor lay truecol)
    )
   )  
 ); vlax
)