Problem is, we see dark things and assume they are black things. When, in reality, it’s very hard to find something that is pure black. Roads aren’t black. Your office chair isn’t black. The sidebar in Sparrow isn’t black. Words on web pages aren’t black.
This infographic is a couple years old now, but still holds true.
We’ve encountered lots of this theory (often without realising) through planning blog content, writing help articles, designing and building landing pages, fixing customer tools. The principles even apply to non-technology stuff – “meeting the parents”, for example. 😉
It’s quite all-encompassing.