Kinda depends on a lot of things- what phone you are currently using (and what launcher), what version of Android, etc, etc...
There is no toggle for it, however. It's either on or off. It's not due to wallpaper color (that would affect the newest Google Launcher's automatically theming engine, but not much else).
The Google Play Store is feeling blue - Play Store’s accent color has changed from green to blue (www.androidpolice.com)