GASの予定の色変更は色番号ではなくカラー名を使用したほうが良い

function myFunction() {
  const calender = CalendarApp.getDefaultCalendar()
  const now = new Date()
  const addZero = (n) => ('0' + n).slice(-2)
  const [year, month, date] = [now.getFullYear(), addZero(now.getMonth() + 1), addZero(now.getDate())]
  const startDate = new Date(`${year}/${month}/${date} 12:00`)
  const endDate = new Date(`${year}/${month}/${date} 13:00`)

  const event = calender.createEvent(
    '昼休み',
    startDate,
    endDate,
  )
  event.setColor(CalendarApp.EventColor.PALE_RED)
}