Is it possible in any way to reduce the distinct colors in an image with the result being visually similar as possible to the original?

I have found out that I can achieve this by setting the palette and bitdepth parameters when encoding to PNG format.

The question I am asking myself is whether this is also possible outside or before the encoding process and without committing to a certain image format. And if this is the case, is it also possible to reduce to any number of colors instead of fixed bit depths?

Thanks in advance.