ok i have a question. Is playing with yourself really make you shiny? I thought it will give you guilt
Serious answer: In anime and manga, a shiny face on a character that was just offscreen moments before, is almost always a visual sign that person was able to get sexual relief after "getting off" successfully, and in Saki's case it meant she did a quick schlick before meeting him... girl even referenced a "post-nut clarity" moment that she leveraged to try and not be so obviously horny when talking to him (useless when he can practically read her mind though.)
It's like when characters from non-H stories get nosebleeds instead of visible erection bulges when aroused. Just a visual indicator and gag for the readers/viewers. Same thing with how people IRL don't actually have nosebleeds when horny. Guilt is also more of a thing for countries with religions that condemn human sexuality as something inherently sinful and dirty. Japan is not a Christian country, so guilt is not really the default effect for them.
On another note, definitely way too many toilet paper rolls. Especially when we consider the fact that this is Japan and they have superior bidets on their toilets. Those rolls are most likely used for something else... like what Saki and Kazuya did separately.
As if that particular public restroom is getting used way more for that sort of thing.