system
Given the following post, respond as if you're commenting on Reddit:
human
How do you "make" color?

Sorry if this question is kind of stupid, I've just always wondered. If you get blue paint, what makes it blue? I know color has to do with light and all, but what do you do/mix to get that blue color? And if its maybe a blue mineral, where does that color blue come from?
gpt
Paint gets its color from pigments, i.e. colored stuff. When you say something has a certain color it means it's absorbing light of some color and turning it to heat, or re-emitting it but with a different color. The light absorbed is the 'complementary' color to the color you see, since ordinary light is white and has all wavelengths of light (colors) in it. For instance, if something appears yellow, it's because it absorbs blue light, since blue is the complementary color to yellow.

The reason why stuff appears colored, then, is because things don't absorb or reflect light equally at all wavelengths. The reasons for that, in turn, are pretty complicated. It depends on the chemical composition (what elements and molecules make up the stuff) but also how the molecules are organized in the material, and how the material is composed at the larger scale. E.g. a single salt crystal is translucent, but granular salt is white - because it's broken up into many crystals with many surfaces, each of which reflects some light. 

Corundum is a mineral made of aluminum oxide - a translucent substance that covers just about all the aluminum you've ever seen (aluminum forms a thin oxide layer instantly on contact with air). But just the slightest impurities in the corundum of various metal atoms will change its color. If it has traces of chromium, it'll be red and that's what you call a ruby. If it has some titanium as well, it's a sapphire. The fact that these are small (less than 1%) impurities tells you something about how subtle these things can be. 

Single molecules can have a color as well, at least if you have a well-defined set of surroundings (now that I've established the surroundings are important). So for instance you can say that various compounds have various colors when dissolved in water. But you can't _quite_ say that the molecule _itself_ has that color. It may very well have a different color as a gas or solid, for instance. 

Even isotopes - atomic nuclei of the same element but different mass, can have an effect on color. Which is rather impressive, since it has almost no measurable effects on chemistry in general. For instance, pure water is (very weakly) blue in color. But heavy water (where hydrogen atoms are changed for heavier deuterium ones) is colorless.

So in short, many many things can cause color.