Depends on the use. If changing the color of an object, you could grab the existing AxCmColor and change that. For example, setting magenta by ACI:
(defun setMagenta (obj / color)
(setq color (vla-get-TrueColor obj))
(vla-put-ColorIndex color acMagenta)
(vla-put-TrueColor obj color)
(vlax-release-object color)
)