Thanks for the GIF - It goes by too quickly. I know how to annotate in all colors other than white.
After you bring up the text window, you are doing something to make the white color choice button show up. I do not get that choice. I see you clicking on a white button twice but no luck here either on my iPhone or my iPad iPhone SE 1st version 14.4 iPad 14.2
The best I can do is to click on the multi color button and then slide all three sliders to the right. At that point the X on the top right does nothing. I just now figured out to slide the whole screen down and finally the text is white. I have to adjust te sliders every time. Your version looks more direct than that. For not at least I have a work around. Thanks