To anyone with an iPhone X that had an earlier model before it, is the screen colour supposed to be off?
With TrueTone off, everything is tinted blue and with it on everything is tinted yellow. I compared it to my iPhone 6S and the 6S colour is perfect, but unless I look at the screen from a 45* angle, the colour is always off. Is this normal, and are other people's phones like this?
There is no "perfect" screen color tint. A screen that appears perfect outdoors will appear to have a blue tint indoors. A screen that looks perfect indoors will appear to have a yellow tint outdoors. You can prefer one or the other, but there is no absolute best.
The "proper" color tint (while point) is determined by your brain, which takes everything in your field of view and averages it to determine what color white should be. Since it depends on what's in your field of vision, it is constantly changing, and it's impossible for a device to match it all the time. Indoors, your brain's white point will skew towards yellow/red due to indoor lighting. Outdoors it will skew towards blue due to sunlight and blue sky.
The iPhone X's white point tests out to 6700K in reviews, which is just slightly blue. 6500K is "white" for sun + blue sky. 5600K is "white" for only sunlight.
The iPhone 6 screen was tinted heavily blue at 7400K. You normally only get 7000K-10000K outdoors in the shade, where most of the light you're getting comes from the blue sky. So if you're using that as your reference for "white", then yes the iPhone X screen will appear yellow with TrueTone on. But as most color professionals work with a white point of 5500K-6500K, the iPhone X's white point is more "accurate".