Q:

setBackGroundRGB not accepting string

The class setBackgroundRGB() works if I pass it a  literal
setBackgroundRGB(255,255,255); 

but if I pass it a variable instead, it fails:
_Color = "255, 255, 255";

setBackgroundRGB(_Color); 

Does not work and returns an error
`Cannot find method setBackgroundRGB(string)`

Do i need to do some kind of conversion here that I am not aware of?

A:

Indeed the API does not have a method setBackgroundRGB(string), provides a method setBackgroundRGB(Integer, Integer, Integer), however, an option to achieve what you need, having a string as input is:
function setColorToRange() {
  var colorRGB = '0, 255, 0';
  var ss = SpreadsheetApp.getActiveSpreadsheet();
  var sheet = ss.getActiveSheet();      
  var range = sheet.getRange('A1:B3');
  range.setBackgroundRGB.apply(range, colorRGB.split(', '));
}

UPDATE
To get the vector can be applied several improvements, widening a little the example presented, we integrate some of the improvements indicated in the comments, resulting in the following:
function setColorToRange() {
  var colorRGB = '0, 255, 0';
  var ss = SpreadsheetApp.getActiveSpreadsheet();
  var sheet = ss.getActiveSheet();      
  var range = sheet.getRange('A1:B3');
  var arrColorRGB = getRGB(RGBString);
  range.setBackgroundRGB.apply(range, arrColorRGB);
}

function getRGB(RGBString) {
  // Returns a vector of integers
  return RGBString.replace(/ /g, '').split(',').map(returnInt);
}

function returnInt(value) {
  return parseInt(value, 10);
}

