system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 Why do some colours go well together and some colours not?
gpt
Some part of it is seeing which color combinations we're already familiar with. For example, if you were to pair leaf green with almost any shade of red, pink, yellow, orange, or purple, they would work together. It's a very common set of color combinations in fashion, interior decor, and graphic design. We also have all seen it (assuming you're not color blind) in nature, as flowers have these color combos with their leaves. Green is practically a neutral to us. For a similar reason, navy blue is considered a pseudo neutral color that matches with almost every other color; it's also the color of the night sky and the ocean. We've all seen it many many times, so seeing it again is comfortable. 

The color combo of red, yellow, and blue is not inherently childish, but some people associate it with preschool or elementary school related things, because it's often used for kids' stuff in marketing and product packaging. 

Basically, we become familiar with certain color combos, and sometimes we start associating them with certain things because we see them in a specific context so many times. If we like that context, we are more likely to like that combo. If we don't like that context, we are less likely to like the combo.