Okay, then "s/sky is blue/water is wet/g" or "s/sky is blue/fire is hot/g"
Edit: I can't just let this lie. Just because you can't see it doesn't mean it's not true. The sky at night is still blue, there's just not enough light for human eyes to see it. I have plenty of footage from night skies where the sky is still clearly blue. This footage [0] is clearly taken at night while the moon is below horizon then the sky becomes blue again (still at night) when the moon rises. The light reflections in the water as well as still being able to see the stars in a blue sky shows the sky is still blue even at night.
https://www.justice.gov/agencies/chart