Can I use SIPS to reduce the size of PNG files by reducing the number of palette colors?

I need to shrink [PNG](http://www.libpng.org/pub/png/book/chapter08.html) files like the ones generated as the default macOS screenshot. [SIPS](https://developer.apple.com/legacy/library/documentation/Darwin/Reference/ManPages/man1/sips.1.html) was my first stop since it's faster than any other solution but I haven't quite been able to find the right command. This is my starting point to reformat to PNG: /usr/bin/sips -s format png "$f" --out "${f%.*}.png" Which options are needed? --setProperty bitsPerSample Bits/sample looks promising but I'm not sure what to enter here.
Does 32 mean 32 colors in this context, for example? Can I use SIPS to quickly reduce the number of colors to shrink file size?