Google Pixel XL features a dedicated sRGB display color mode, which makes the screen display colors that comply with the sRGB color standard, which is what most images on the web are taken with. The Pixel AMOLED display will display colors which are more saturated and better please the eye of the viewer, but it doesn't necessarily mean that they will be accurate. If you do want to view the colors as accurate as possible and those which better adhere to the original scene, you need to tell Pixel XL to use the sRGB display color mode.
The Pixel XL AMOLED display can display 97.6% of the Adobe RGB spectrum, which is much wider than the sRGB one and because Android doesn't have any built-in color management and can't properly utilize color spaces larger than sRGB, this is why you see a significantly more saturated rendering. According to CALMAN 5 Custom ColorChecker test (sRGB) which measured general color accuracy, the Google Pixel is performing quite poorly with a score of 5.65, whether the Google Nexus 5X has a much better score of 2.77 (lower is better) and the iPhone 6s scored 2.91. The Pixel XL also performed poorly in the Saturation accuracy.
That's of course with the SRGB mode turned off. When you turn it on, the Pixel XL outperforms all the other screens in terms of color accuracy according to a test done by techspot.com using CALMAN 5 accuracy test, passing all the other screen with an amazing dE2000 value of 1.18 - superb color accuracy.
Don't understand it wrong. The Pixel XL screen was design by default to present more vivid rendering, and most people will prefer it like that. It just if you want to view images with better color accuracy, you need to turn this features on, and you can rest assured that the Pixel XL is one of the most accurate screens out there with that sRGB mode turned on.
So how do you turn the sRGB mode on? You do this by accessing the Settings, then choose Developer Options and then Picture color mode. Then you just need to toggle this option, and you should see images with different colors than what it would look like had you chosen not to enable this feature. Colors will now look more natural. For some people, the colors might look more boring and flat, but this how the image looks without messing around with the color reproduction to create a more vivid rendering.
I personally like the look of the image with that sRGB picture color mode turned off. Keep in mind that it doesn't change the image file, just the way the screen renders the color data from the image file. Also if you find images looking oversaturated and are not to your taste, you can use this mode to tone down the color and make them look more realistic. It's a subjective obviously, so just try it out and see whether you like it when this feature is turned on or off.