How do you shine light on something vs through something? And I agree that the whole complementary color thing doesn't tell us much about the light inherently but it still seems useful (or at least interesting) to determine what color we perceive things to be.

Thank you for elaborating! So the color of something is mainly based on what color of those that we can see that is absorbed least, pretty intuitive. I'm assuming there are also times when we would have to take combinations into account if there are a few wavelengths that are not absorbed much? Also I was reading a bit more about complementary colors and I think the premise is that 2 complementary colors would combine to form white. So I guess the idea is that if one color is absorbed much more than others, we would interpret the remaining light as the complementary color to the one that was absorbed, since the light was initially white and lost a lot of light of a specific color.

And 1 more question, does an emission spectrum for some molecule give essentially the same information as an absorption spectrum?

What I'm confused about is why are some absorption spectra shown as graphs with a continuous line? I was under the impression that only specific light with one or more specific energy, frequency, and wavelength could be absorbed by a given atom/molecule. Also, how do we find what color something appears to be from it's maximum absorption? For example, water absorbs red more than other visible light, so it's absorption maximum that we can see is red. How do we know from this that water is blue? I know there are complementary colors but I'm confused on how we actually got those. I made a post here recently asking basically this but it hasn't been put up yet.