The Roman History · Amm. 20.11.28

And it is so tempered with this mixed beauty, as mankind believe, because its first portion is discerned in a thin diluted state, of the same colour as the air which surrounds it; the next line is tawny, that is a somewhat richer colour than yellow; the third is scarlet, because it is opposite to the bright rays of the sun, and so pumps up and appropriates, if one may so say, the most subtle portion of its beams; the fourth is purple, because the density of the spray by which the splendour of the sun's rays is quenched shines between, and so it assumes a colour near that of flame; and as that colour is the more diffused, it shades off into blue and green.

Ammianus Marcellinus · translated by C. D. Yonge · 1911 reprint
C. D. Yonge translation (preface dated February 1862); G. Bell and Sons, 1911; Project Gutenberg #28587
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