In ExtendScript on Windows only, adding any InDesign page item after adding a color will delete the color:

var doc = app.documents.add();
// adds black CMYK color
sw = doc.colors.add();
// this line will delete the color that was just added on Windows only
var rect = doc.rectangles.add();
alert(sw.isValid);


This actually manifested for me as a crash bug. This script crashes InDesign 2024 Windows:

var doc = app.documents.add();
var sw = doc.colors.add();
doc.rectangles.add({fillColor: sw});

This crashes because the last line deletes the swatch when it adds the rectangle, then tries to use the swatch.