What is the colour of fluorine?

Fluorine is known to be a very reactive, poisonous yellow gas. This characteristic is important because it highlights the element's chemical properties, particularly its high reactivity, which categorizes it within the halogens. The distinct yellow color is a key identifying feature that sets it apart from other gases, particularly because many gaseous elements are either colorless or have less pronounced colors.

In the context of other options, it's relevant to note that while some gases can appear green or red, these descriptions do not match fluorine's properties. The description of fluorine as a colorless gas is incorrect, as its yellow coloration is observable under standard atmospheric conditions. Additionally, the dark red gas does not pertain to fluorine but rather to other substances, such as bromine in its gaseous state. Thus, option C accurately encapsulates the essential qualities of fluorine, making it the right choice.