What do the lap colors in the iOS stopwatch mean?

Solution 1:

The green lap is the fastest lap, i.e. the shortest duration of any lap.

The red lap is the slowest lap, i.e. the longest duration of any lap.

If you use VoiceOver or Speak Screen to verbalise the content, or other accessibility functionality like a Braille keyboard, these rows are read out with the suffix ‘fastest’ and ‘slowest’ respectively after the time.