G'day Bruce,
I'm not familiar with St. Augustine lawn, but in general
this sort of pigmentation is a protective reaction to sunshine. It's like
you or me getting a 'bit of a tan up'. Some plants can also mirror this
reaction with cold weather, but with the heat we have been having that is
not likely right now. Your plants are 'hardening off', as they say, and is
totally natural and not in itself problematic.

China
Wingham
NSW