In the same book ” Principles of Color Design” Wong notes that numerous steps of gray are possible but it is simpler to arrange them into three groups:

The dark gray series- extremely dark gray ( %90 blackness) ,very dark gray( %80), dark gray ( %70)

The middle gray series-dark middle gray( %60), middle gray( %50), light middle gray ( %40)

The light gray series-light gray ( %30), very light gray( %20), extremely light gray( %10)

These nine steps provide a basis for an accurate color specification. It is a guide to visual thinking. Judging values with the naked eye can be inaccurate since we tend to distinguish more gradations in the range of light grays than dark grays.