问使JavaScript与readOnly属性同时设置背景色时遇到困难

EN

<input type='button' id='BtnEdit' value='Edit' onclick='ThisEdit(this)'/>

function ThisEdit(event){

    var eid = event;
    var btnlabel = document.getElementById("BtnEdit").value;

    if(btnlabel == "Edit") { 

        document.getElementById("BtnEdit").value = "Save";

        document.getElementById("NT").readOnly = false;
        document.getElementById("ND").readOnly = false;
        document.getElementById("NU").readOnly = false;     

        document.getElementById("NT").style.backgroundColor = "White";
        document.getElementById("ND").style.backgroundColor = "White";
        document.getElementById("NU").style.backgroundColor = "White";      

    }

    if(btnlabel == "Save") { 

        document.getElementById("BtnEdit").value = "Edit"; 

        document.getElementById("NT").readOnly = true;
        document.getElementById("ND").readOnly = true;
        document.getElementById("NU").readOnly = true;

        document.getElementById("NT").style.backgroundColor = "Gray";
        document.getElementById("ND").style.backgroundColor = "Gray";
        document.getElementById("NU").style.backgroundColor = "Gray"; 

    }
}