Google Sheets: For each cell in a range, with a given background color, count or sum it.

CountCellsByBgColor.gs

/**
 * Google Sheets script: countCellsByBgColor(inputRange, colorReference, operation = 'count')
 * For each cell in a range, with a given background color, count or sum it.
 *
 * @param {String} inputRange The range of cells to check for the background color.
 * @param {String} colorReference The cell with the background color to count.
 * @param {String} operation [count|sum] The operation to perform over selected range.
 * @param {Int} eachRow The number of rows to jump to at each interaction loop.
 * @param {Int} eachColumn The number of columns to jump to at each interaction loop.
 * @return {Number} The number of cells with a matching background.
 */
function countCellsByBgColor(inputRange, colorReference, operation = 'count', eachRow = 1, eachColumn = 1) {
  var sheet = SpreadsheetApp.getActiveSheet(),
      range = sheet.getRange(inputRange),
      color = sheet.getRange(colorReference).getBackground(),
      numRows = range.getNumRows(),
      numCols = range.getNumColumns(),
      c = 0;

  for (var i = 1; i <= numRows; i = i + eachRow) {
    for (var j = 1; j <= numCols; j = j + eachColumn) {
      var cell = range.getCell(i,j),
          bgcolor = cell.getBackground();
      
      if (bgcolor == color) {
        switch (operation) {
          case 'sum':
              c += parseInt(cell.getValue());
            break;
          default: //count
            ++c;
        }
      }
    }
  }
  
  if (c) {
    SpreadsheetApp.flush();
  }

  return c;
};