First of all, reverting your edit wasn't a challenge to whether or not the information was correct. It was purely based on the lack of a citation. Wikilinks are not acceptable substitutions for inline citations. Each article must be able to stand on its own set of references and not rely on a wikilink to another article's reference. Refer to WP:CITE for more information. Secondly, comparing the flags and drawing a conclusion – no matter how reasonable and obvious it might be – can be considered a form of original research which is not permitted on Wikipedia. Citing a reliable source that supports the analogy alleviates the concern. It also helps us establish the significance of the information as it relates to the topic. Without that verification, it may be a trivial observation that doesn't necessarily belong in this article (see WP:DUE). As for your "sky is blue" analogy, it's an apples-to-oranges comparison. Take for example the statement that "the Eiffel Tower is in Paris". This is widely accepted fact that doesn't require research, and therefore doesn't require a citation. However, the statement that "Gustave Eiffel designed the Eiffel Tower" would need a citation, although it is easily verifiable. It may be hard to see the difference, but the rule of thumb is when you think it may be challenged, it's best to err on the side of caution and include a citation.