There are plenty of things we take for granted. One is that most plants are green. Find out why that might not be true on other planets.
The grass is always greener on the other side of the fence. Unless you're a bird or a bug - in which case the grass might be any color at all. Physics and biology band together to color our perception of the universe. First physics kicks in - plants absorb red and blue visible light in order to power their cells, leaving green to be reflected back at us. We see the green light, and conclude that the plant is green. But we don't see what else the plant reflects. Biology left us with eyes that can only take in a certain spectrum of light - from violet to red. Wavelengths longer than red or shorter than violet miss our eyes entirely. Bugs and birds, however, can see ultraviolet light bouncing off the plants around them. What we see as green, they may see as a completely different color.
Red and blue drive the reactions in plants on earth, but plants on other worlds may not have the luxury of picking and choosing wavelengths of light. Plants on worlds that have multiple near stars may be inundated enough by UV light that need to block it - making leaves shine to earth-bird or earth-bug eyes. Meanwhile, worlds that orbit a relatively poor source of light, like a red dwarf, may have to develop pigments that absorb every kind of light from infrared to UV in order to gather enough energy to survive. A healthy forest orbiting a red dwarf may be a rich, lustrous black.
Clearly, there's no way to know what plants are actually out there in the universe. Still, it's fun to think of the many ways life can evolve when shaped by alien conditions, and the ways our perception of the world can differ from other species, even when the mechanisms of perception work the same way. Who knows what could be out there - and why it could be the way it is.
Top image via Royal Astronomical Society. Black plants from Earth photo by J O'Malley-James