The further Android comes, the more it seems Google focuses on security. Luckily, this hasn’t much affected those who modify their devices, and this will be no different. Google has posted a new support page related to an upcoming verified boot system, which provides warnings about security when booting.
There are currently three warnings that Google has shown us. The yellow warning means that the device software has been changed, the orange warning means that the bootloader is unlocked and can’t check the software, and the red means something has been seriously modified. These warnings will pop up either due to intentional or unintentional changes, but custom ROMs are guaranteed to set them off.
If you’ve purposefully modified your software, don’t worry about the warnings. They disappear after 10 seconds, and hitting the power button dismisses them immediately. This verified boot system will likely be added to Android M, which is coming out soon.