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The original color designation for your hull was "desert tan". Whaler has produced several variations on this shade over the years, but I'm pretty sure the color was consistant for all Rockland-built "classics" between the mid seventies and the late eighties.
Unless another poster has a direct answer or link for you, I would suggest calling a big Whaler shop (Nauset Marine, e.g.) and asking them about codes and sources for paint.