Our capacity to name colours in all their rich and detailed variegations is surprisingly limited, and so the recourse to the formula of ‘something-like colour’ is common and delivers some of the missing hues. Thoreau is a meticulous observer of nature and beauty, and takes great care to tease out minute distinctions.
Try coming up with a name for the colour of ‘slate-color water’ – prize for the best one.
Source: Henry David Thoreau, The Journal 1837-1861, Damion Searls (ed.), preface by John R. Stilgoe (New York: New York Review Books, 2009), p. xi