During night engagements the flash produced by ignition can reveal the location of the gun to enemy forces and cause temporary night-blindness among the gun crew by photo-bleaching visual purple.
From the en.wikipedia.org
So the color purple is a visual key.
From the inrich.com
More delicate but worth adding to a shady spot is the aptly named Viola odorata, its delicate purple flowers adding a visual treat.
From the sfgate.com
The golden yellow of the daisies and purple from the gomphrena created visual excitement in the garden causing everyone to stop and take photographs to help inspire.
From the charlotteobserver.com
The visual theme of Xiao Xiao, a plain black stick-man, fighting against a purple stick-man Boss, remains constant throughout the series.
From the en.wikipedia.org
Mounds of purple wild geraniums and the quick arrival of the wide, cold South Kern River made the early morning a visual feast.
From the ocregister.com
These pretty flowers are then joined by the additional visual treat of flat bean pods shamelessly flaunting themselves in a screaming, electric purple not seen since the days of disco.