Splitting a post, how to change text after “more” link has been clicked

I’ve been searching like crazy for this, anyone that can point me in the right direction it would help a ton. I want to split my posts with the “read more”, which I can do fine. But when someone clicks “read more” and it shows the whole post, I want the text that was before the “read more” link to them change to a different color (or be italicized or something). Any help is much appreciated!