Unobtrusive

un.ob.tru.sive

1. not obtrusive : not blatant, arresting or aggressive : inconspicuous

un.ob-tru.sive.ly (adverb)

un.ob.tru.sive.ness (noun)

To put it to use:

They were dressed in unobtrusive fashion.

The first known use of the word unobtrusive was in the year 1743.

See also: 

discreet, unnoticeable, inconspicuous, invisible.

I wonder, is this unobtrusive enough? Paparazzi spotted Brangelina, anyway.