Range not found error on user-defined function that use a cell reference as argument

I’m writing a UDF and get an error. From the documentation, it seems straightforward. My goal is to check the background color of a cell. The code is shown below and the calling format is “isColor(cell)”. I get an error (Range not found (line 5, file “Code”) on the line var cell = ...

I tried to pass the cell address as a string by making the calling format isColor(cell("address",A2))

function isColor(input) {   var ss = SpreadsheetApp.getActiveSpreadsheet();   var sheet = ss.getSheets()[0];   Logger.log(input);   var cell = sheet.getRange(input);   var color = cell.getBackground();   Logger.log(cell);   color = cell.getBackground();    Logger.log(color);   var result = false;   if (color == '#ffffff') {       result = false;}       else {       result = true;       }   return result; }