Different operating systems display visual elements very differently. Especially when it's things to do with the shadow DOM (input fields). If you look through the css and can't find anything like this:
@supports (-webkit-overflow-scrolling: touch) {
/* CSS specific to iOS devices */
}
@supports not (-webkit-overflow-scrolling: touch) {
/* CSS for other than iOS devices */
}
Taken from here:
CSS media query target only iOS devices
Then there's a chance it is the operating system/browser that you're using.
So first i would check that you are using the same web browser on each device you're testing on, e.g. visual elements will usually be different between firefox, safari, chrome etc. This will tell you what it is likely to be (os or web browser).
Then i would explicitly set the CSS properties of the input fields, set it to the colours you want for all devices. Failing that, try the media queries above and explicitly set the colours you want for IOS devices.