I aced it
I suspect color adjustment of your monitor, ambient lighting/ surface reflection on your monitor, attention and fatigue would all affect the result.
I beleive the eye gets tired of staring at similar colors and would become less sensitive the harder you try, go and do one of the other strips and come back.
Once I thought I was done I tried blinking rapdily and looking away and back again rapidly (flick gazing) to see if any popped up as out of place, I got one or two this way.