This was actually confirmed by Google themselves, perhaps inadvertently, via the Google Nexus Twitter account who responded to a question posed by a user. In their response, they revealed the dialer which appears to have gotten a new coat of paint. Unlike Calendar, the Dialer app has not received a drastic overhaul.
As you can see in the comparison screenshot above, it’s pretty much the same as the old app, except that the title bar is now colored in blue and the image in the contact has also been changed slightly to look “flatter”, which seems to be a new design language that Google could apply to Android.
The idea that apps could be getting colors in their title bars also seems to be in line with rumors about Project Hera. The project is meant to merge Google’s products and services on the web, Chrome, and Android, thus allowing them to sync and update themselves in real time a lot better. Not much is known right now but perhaps more will be revealed at Google I/O which will be taking place in June.