Putting different id in different paths on my Leaflet map

Leaflet offers no way of assigning HTML id attribute to HTML path elements that represent vector layer feature. But since you just want a way to select desired path elements from the DOM structure, there is a bit hacked way to do that.

One of the Leaflet style options is className, which can be any string. This class name is then added to the path element class list. There is no harm if such CSS class actually does not exist, it will be just "misused" to get collection of desired path elements with the document.getElementsByClassName method.

If in your case you would want to get all elements where feature.color has value 'red', your code could look something like this:

L.geoJSON(polygon, {
  style: function (feature) {
    return {
      stroke: true,
      fillColor: feature.color,
      fillOpacity: opacity,
      color: feature.color,
      opacity: opacity,
      weight: 2,
      className: (feature.color == 'red') ? 'id_red' : '';
    };
  },
})
.bindPopup(function (layer) {
  return layer.feature.properties.name;
})
.addTo(map);

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var red_elements = document.getElementsByClassName('id_red');