As we've stated many times before, Google is pretty big on the safety and security of its users. If it wasn't apparent before from all the efforts they've made, the latest change should help a little to display that Google really does have a strong focus on making Android a safe platform for everyone. Over on the Nexus support page Google talks about a new "warning system" for Android during the boot-up process, which would essentially give users three different types of warnings based on the security level of their device in an attempt to alert them if it is deemed unsafe to use.
Each warning system message shows up during the boot process with its own color to denote the severity of the warning after the operating system has been checked and verified. The first warning system message that users could see is yellow, and as Google describes, this means that the device has loaded up a different operating system than what was shipped with the device. The second warning message shows up in orange, and this denotes that the device/bootloader have been unlocked. With an unlocked device the operating system isn't capable of being scanned for corruption or being verified as safe to use. The third and last warning message shows up as red and could be considered the most concerning, as it means a user's device may be corrupt, and therefore is deemed unsafe to use.
One obvious note worth mentioning, is that these warning messages can pop up for users who tinker with their devices, so if you've unlocked your device and/or loaded up an aftermarket piece of firmware, it's likely you would see one of these messages pertaining to the state of your operating system. In this particular scenario, the messages can of course, simply be dismissed and users can do so either by waiting for the messages to disappear on their own or by hitting the power button real quick dismiss it manually and immediately. If a user sees any of these messages and they hadn't made any of these modifications, Google's recommendation is to not use the device in this state, and not prior to restoring it to a default if it's possible. As these aren't currently part of the Android operating system, there's a possibility these could be a product of the upcoming Android M version of the OS.