2

For example, if I had a dark blue or black background choosing white text will make it stand-out and more readable.

If I had a white or off-white background, black text will stand-out and be readable.

What do I do if the background color is neither dark enough for white to show well nor light enough for black to show well?

3

What you are looking for is contrast

In order to measure contrast there are a lot of ways, I personally use this website. It grades the contrast between colors, and I personally found that grade 5 and above is a pretty good contrast for type and its background

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