"day by day" is not really used that often, so it sounds a little forced. I would say, "The tomatoes in my garden are getting a little redder every day."
When do people use day by day? More commonly when dealing with adversity, or possibly when presenting information for all days of an event. "day by day" My mother is getting better day by day following her operation. You have to take it day by day (You have to take it one day at a time)
We will post a day-by-day listing of times and places for events during the Spring Charity Event. (often with hyphens in this usage)