The scenario can be implemented by utilizing the OnChildRowExpand and OnChildRowCollapse client-side (JavaScript) event. On each event, set the background color of the row and cells to a color as shown in the following snippet code.

function WebGrid1_OnChildRowExpand(controlId, tblName, rowIndex, rowEl) {
    var WebGrid1 = ISGetObject(controlId);

    // use the code below to set the background color of row element
    rowEl.style.backgroundColor = "Yellow";

    // iterates through each cell element and set its background color
    for (var i = 0; i < rowEl.cells.length; i++) {
        rowEl.cells[i].style.backgroundColor = "Yellow";
    }

    return true;
}

function WebGrid1_OnChildRowCollapse(controlId, tblName, rowIndex, rowEl) {
    var WebGrid1 = ISGetObject(controlId);

    // use the code below to reset the background color of row element
    rowEl.style.backgroundColor = "";

    // iterates through each cell element and reset its background color
    for (var i = 0; i < rowEl.cells.length; i++) {
        rowEl.cells[i].style.backgroundColor = "";
    }

    return true;
}

A simple sample of is enclosed as attachment for reference.

Please kindly let us know whether this helps or not.