Red can be a very positive color. From red wine to red roses, I have nothing but good things to point out. Hearts are red, a red-letter day is to be treasured, Major Feasts in Anglican Prayer Books are printed in red--as were "rubrics" originally. "Red-blooded" folk--usually male!--are prized for their courage. Newly-minted cardinals are given a "red hat".
Yet red is also the color of danger, as in a red flag and red alert. We stop our cars at a red light. A "red light district" holds houses of "ill repute". When we are ashamed or embarrassed, our flushed cheeks betray a red face. Red is the color of Communism, not usually welcome in the circles most of us move in. "Red Ink" refers to a loss in business, etc. We should try to avoid going past the red line on instruments, such as a speedometer. "Rednecks" is not a positive description. We avoid "red-eye" flights if we can.
There is ambiguity, even if sound advice, in the old proverb: "Red sky at night, shepherd's delight; red sky at morning, shepherd's warning". In the Bay Area, Stanford supporters love red--but don't wear that color to a Cal game!