White Aster turning red

My white aster has bloomed really well with lots of beautiful white flowers. But lately, the flower petals are having a slight red tinge on it. Is that normal?

When I bought the white aster, the tag says it blooms for only a season. A quick check on the internet reveals it's a perenial. So which is which?

Do I still need to deadhead the red tinged flowers to encourage new growth at this time of the year or is it too late now?

I always think this is the most confusing thing about gardening, you can get a perennial hardy Aster and a Tender Aster. If yours says for one season it is most likely the tender one.

The red tinge is not unusual plants often have small variations. I am still dead heading my plants in the hope that we get a warm September.