On most political maps, each country is generally given a specific color, e.g. England/Great Britain is red, France is blue, Germany is dark grey, pre-Soviet Russia is dark green, Spain is yellow, etc. When and how was this decided?
There are no standard map colors for countries, as a quick search of historic maps will reveal. A search of contemporary images demonstrates the same thing.
Political maps typically have four (sometimes five) colors that are used to distinguish adjacent territories from each other. Though mapmakers had known for decades that this nearly always worked out fine, meaning no territories of the same color ended up adjacent, it wasn't until 1976 that it was proven mathematically.
When color is added to maps, red tends to come to the visual foreground and appear the most prominent. It also was the color most frequently used for two-color printing. So English-language maps, with the majority of 19th century examples originating in Great Britain, often used red to depict the United Kingdom and its empire.