Change color in tables via "Find/Change"

I'm assuming you do not have a cell style applied to those cells? (That would be the ideal way to set up these types of tables in the future.)

The following script will find all cells with a background tint of 40% (no matter what color) and apply the color swatch called "Beige". So before you run this script, you will need to make sure you have created a "Beige" color swatch, and do not save it inside any swatch folder.

app.doScript(Main, undefined, undefined, UndoModes.ENTIRE_SCRIPT,"Run Script");
function Main(){

    //Declare variables
    var myDoc = app.activeDocument;
    var beigeSwatch = myDoc.swatches.itemByName("Beige");

    // loop pages
    for (var pageCount = 0; myDoc.pages.length > pageCount; pageCount++) {
        var myPage = myDoc.pages[pageCount];

        // loop textframes
        for (var textFrameCount = 0; textFrameCount < myPage.textFrames.length; textFrameCount++) {
            var myTextFrame = myPage.textFrames[textFrameCount];

            // loop tables
            for (var tableCount = 0; tableCount < myTextFrame.tables.length; tableCount++) {
                var myTable = myTextFrame.tables[tableCount];

                // loop cells 
                for (var cellCount = 0; cellCount < myTable.cells.length; cellCount++) {
                    var myCell = myTable.cells[cellCount];

                    //If cell fill color is black, apply new fill color
                    if (myCell.fillTint==40) {
                        myCell.fillColor = beigeSwatch;
                        myCell.fillTint = 100;
                    }
                }
            }
        }
    }
}

If your 40% grey is actually saved as its own swatch (with the tint percentage defined as part of the swatch), I'll need to adjust the script. Just let me know if this doesn't work!