Android 5.1 is still only available on a limited number of Android One phones, but a few interesting tweaks are already on display. One feature that seems tailor-made for Android One has to do with the dual-SIM capabilities. Android 5.1 includes the ability to set a different dialer theme color for each SIM so you'll know which one you're using to place calls.
There was no native support for dual-SIM features in Android 4.4, so any previous implementations were the doing of OEMs. There are a few devices with similar features, but they don't look nearly as nice (ex. Acer Liquid Z500). Android 5.0 has some dual-SIM goodies, but it doesn't look like the themes are included. On 5.1, you can access the color options in the SIM card menu of the main system settings.
We won't know for sure if this is a stock Android 5.1 feature or something exclusive to Android One until 5.1 is in AOSP and we see it installed on dual-SIM phones that aren't part of the One program. For most phones, this feature probably won't be exposed anyway.
- Ramit Suri