Holding at a 45 degree angle and against a white background, regarding example a tablecloth, look down into your glass to watch the rich colour of the wine bottle.
Tilt the glass 45 degrees and hold a white paper on the inside background. https://zenwriting.net/sofabee77/mot-loai-cognac-dem-den-cam-giac-say-dam-manh-me-no-the-hien-su-song-dong