As OLED is an emissive display technology, an OLED pixel only draws power when it is used - and the brighter it is, the more power it consumers. This of course means that adopting a dark UI is better for your device's power life.
During Google's recent Android Dev Summit, the company detailed the power consumption of the OLED display used in its 2016 Pixel smartphone (an SDC FHD 5" AMOLED). As you can see in the image above, the power consumption is different for each color (with red being the most efficient color and blue the least efficient). Surprisingly the display draws quite a bit of power even when completely black.
Google is still not taking advantage of this in all of its designs, but the company is offering Dark Mode for both its YouTube and Android Messages applications (it is also starting to test a dark mode for the Phone application). In the image below you can see just how much power you can save with YouTube's dark mode - around 60% in full brightness.