The sky is currently blue, but it will not be blue forever. There is no universal unchangeable constant which guarantees the colour of the sky as blue. Hell, changing the colour of the sky is something we could do with our current level of technology (it would kill all life on earth in the process, but that is besides the point). The colour that the sky takes depends on atmospheric composition. That's something which could be affected either by us or an externality within very workable time scales. Even in ways which don't destroy life.

Sure, you can fall back on time scales and say that, locally speaking, certain things are less likely to change in the immediate future. Mountains are unlikely to change place, until a catastrophic tectonic shift moves them. But that concedes that I am right and that there are no universal truths as something that is true until a distant point in time isn't a constant universal truth.