Negative values are blue. . . . The ruddy Betelgeuse presents a fine contrast to Rigel, its bluish neighbour in the constellation Orion. The ancients gave the name Antares to the fixed star Alpha Scorpii on account of its resemblance to the red planet Mars. Among the brightest 30 or so, in which colours appear the most distinct, the only definite hues are yellowish oranges, yellows, and pale blues.
True reds appear only in a few objects visible with optical aid, principally carbon stars.
The vibration spreads out the image across the field of vision, blurring it and making the colours more easily visible. Stars should be near the meridian, and therefore, highest in the sky. The eye's colour-perception threshold is 5 or 6 times fainter than the brightness of the sky at Full Moon. It is a much darker hue than the daytime's sky because the brightness is much lower. Common sense dictates screening the Moon from view.