Try to loop through all objects selected and adjust the color based on a variable.

it can be dangerous (but in general it's just bad practice) to try to loop all of the items in a document. Because of the way the artwork tree is, and the way each node in that tree is technically a pageItem, you may end up processing way way more things than you expected (as an example, if you have a compound path, that's one pageItem. But that compound path could be made of dozens of other compound paths and pathItems and groupItems.. You don't want to process all of the children of a compound path. you only need to touch the first path to get and set colors).

Can you share the document you're trying to recolor? What does it contain? Lots of different kinds of art? Or just simple shapes with a simple fill?

For the time being, I'll just show you the loop assuming you're just looping the selected items. But we can discuss better ways to target your artwork if you want.

So, your logic up there is good. You just need to wrap it inside of your loop and target the specific piece by index then you're good to go. it looks like this:

//i always wrap my code in a function to avoid global variables
function setBlackOrWhite()
{
    //make sure there's at least one document open
    if(!app.documents.length)
    {
        alert("Please open a document and select at least one pageItem first.");
        return;
    }

    var doc = app.activeDocument; //make sure to use "var" when declaring a variable or it will be placed in the global scope.
    var sel = doc.selection; //get the selection
    if(!sel.length)
    {
        alert("Please select at least one pageItem and try again.");
        //no selection. exiting script.
        return;
    }

    
    //set up the color variables outside the loop so we don't have to keep re-declaring them
    var whiteFillColor = new RGBColor();
    whiteFillColor.red = 255;
    whiteFillColor.green = 255;
    whiteFillColor.blue = 255;
    var blackFillColor = new RGBColor();
    blackFillColor.red = 0;
    blackFillColor.green = 0;
    blackFillColor.blue = 0;

    //declare some variables that will be used inside the loop.
    //declaring new variables inside of a loop body has been known to cause problems
    //with extendscript. I don't believe this is an issue for regular javascript,
    //but something we need to keep an eye out for, for sure.
    var curItem, colorSum;

    //now loop the selection
    for(var i=0;i<sel.length;i++)
    {
        //within a loop, use your loop variable to access the selection index.
        //just like your original code had "doc.selection[0]". zero is the index.
        //in the loop, we're replacing a hard coded number for a variable that represents
        //all integers between 0 and the length of the selection.
        curItem = doc.selection[ i ]; 
        colorSum = sel.fillColor.red + sel.fillColor.green + sel.fillColor.blue;

        if ( colorSum >= 500 )
        {
            sel.fillColor = whiteFillColor;
        }
        else
        {
            sel.fillColor = blackFillColor;
        }
    }
}
setBlackOrWhite();

Hi @Kernzy 

Below is the same version of your script, just running on all pageItems of the documents. Assuming your script is working fine, I have just added the loop to traverse all pageItems of the documents.

doc = app.activeDocument;

for (var i = 0; i < doc.pageItems.length; i++) {
    var sel = doc.pageItems[i];
    var colorSum = sel.fillColor.red + sel.fillColor.green + sel.fillColor.blue;

    if (colorSum >= 500) {
        var whiteFillColor = new RGBColor();
        whiteFillColor.red = 255;
        whiteFillColor.green = 255;
        whiteFillColor.blue = 255;

        sel.fillColor = whiteFillColor;
    }
    else {
        var blackFillColor = new RGBColor();
        blackFillColor.red = 0;
        blackFillColor.green = 0;
        blackFillColor.blue = 0;

        sel.fillColor = blackFillColor;
    }
}

Let us know if this works for you or not. If not, then it will be good to share the sample documents to see how items are arranged in your documents.