Card and Appbar backgrounds change color on iOS

Hi,
I have a strange issue on when running my app on iOS.

I have a card which has white background and elevation of 4.
When I run the code on device or emulator, it shows a Greyish color (like this).

Secondly, I am using Appbar and setting ite properties as:
Collapsible > Floating > Snap

Now whenever the appbar hides at the top and then reappears when I scroll down, its color also changes from White to this Grey color like the card:

Any idea what can I do to fix this?