Complex sentence: The important issue is not color, but lightness and darkness: a sign with very light gray letters on a charcoal gray background would be acceptable, but a sign with red letters on a black background would not.
Split into simpler sentences: The important issue is not color, but lightness and darkness. Thus, a sign with very light gray letters on a charcoal gray background would be fine, but a sign with red letters on a black background would not.