Look at this...
The shirt on the left is one that bear started wearing last year, around September 2014, possibly as late as Christmas 2014. The shirt on the right is one that has been washed for the first time and is new. They are the same shade of white and I am so proud of myself.
I have had so many washing disasters, so many fails, but when I saw them hanging together looking equally clean I felt that I had actually had a win. Most of last year's shirts survived equally white as well. Bear obviously did his bit with pens, paint and mysterious brown stuff that would not wash out, but there has been a good survival rate.
I don't have a secret. I could say that always doing a separate white wash and using white vinegar as fabric softener is the key. I could praise Ariel, my soap powder of choice. I could mutter about appropriate temperature and line drying. I suspect that the reason that the two shirts are the same shade is luck.
Even though I still can't get the mysterious blue mark out of bear's otherwise brilliant white t-shirt (not shown) I feel pretty good and I'm holding on to that.