When the iOS application starts, the startup image is displayed in full screen at boot time. You have no other options or configuration.
Basically, you will need to provide an image with a white start.
Honestly, flat white PNG will have basically the same file size as transparent (~ 20 kB), so it doesn't matter what color you make it. If the file size really bothers you, you can try using something like ImageOptim to reduce the size of the image.
Apple Human Interface Guide :
Typically, create a startup image that is identical to the first screen of the application.
For convenient and smooth playback, the launch image should look as close as possible to the first screen of your application (obviously, excluding text and dynamic content, but including toolbars, etc.), so it would be better for you to provide a real image rather than a blank screen.
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