If I red line an engine that means I run it at the highest speed before it starts to break apart. When correcting or editing a document I can red line it to highlight changes. The red line can refer to the border surrounding a communist country. Of course we have Red Square the famous parade square in Moscow and the Red Army is the military body in the country of Russia. To see red is to be very angry.
A red tide is a phenomenon where algae become so numerous they discolor the water. Because it is so thick with algae the sea is depleted of oxygen and can be toxic to sea life and to humans. Again, communism is seen as ‘red’ in its political leanings and a move by a large communist army can be referred to as a red tide. Red can be a nickname for a person who has red hair. Such a person with a strong personality is also called a red tide
Red is the colour associated with fire. The iconic firefighter is wearing a red hat and standing beside a red vehicle all dressed to help in fighting fires (though nowadays fire engines are other colours, like yellow). To say something is red hot means it is so hot that it actually has changed colour to red. Imagine a hot poker or a burning log. Here too, there are other colours, but red is always a part of it all.
A red neck could be the result of too much sun without sunscreen or it can refer to a personality of really old fashioned wild west cowboy attitudes. The modern redneck is often a working class white person with a reactionary political attitude. Rednecks are sometimes referred to as provincial, suggesting uneducated. I’ve my redneck moments. They are like when Marie Antoinette said ‘let them eat cake’. Insensitive.
What would Valentines or Christmas be without the colour red? Santa in yellow doesn’t quite do it for me. Red is a colour popular when attempting to stir passion, like in flags or decorations and uniforms. My guess is the red cape of the toreador is more for the audience than to enrage the bull, and that part works. Red is the colour of our blood. It doesn’t get more attention getting than that.