No longer a secret: light-colored status bars in Android M will come with dark-colored icons

As you probably know, Lollipop came with dynamic status bar background colors, which can be changed by the app you're currently running; still, the color of the status bar icons (clock, battery, and other default ones) is fixed and they only appear in white. Yet, this might make them a bit unreadable provided that the background is lighter.
A new flag ("windowLightStatusBar") discovered in the M Developer Preview will enable developers to instruct the Android system to switch the default white status bar theme to a darker one.
Hopefully, developers will embrace this newly-found feature and update their apps with this neat functionality once Android M becomes official later this year.
source: Roman Nurik via Reddit