> I have no idea why they went for this visual, but it's difficult to attribute it to any motivation that is at all desirable.

I think Hanlon's Razor is a better explanation. Look at a wider picture[1][2], and the start of the CBS video I linked above. The building was well-lit, including the doorway area. When Biden starts speaking, the doorway light is turned off, plunging the background into darkness. This was probably done because the lit doorway was brighter than Biden. After a couple minutes, the light comes back up, but dimly. The background is still too dark, but it looks like they only had that one light to work with. Clearly this was being done on the fly.

CNN's color shift at ~12:40, on the other hand, probably had nothing to do with the background. Compare Biden's face before and after the shift. Before the shift, he looks yellow. After the shift, he looks much more natural. The lighting was just bad. CNN decided to adjust for it; CBS didn't.

Meanwhile, photographers love contrast, and the dynamic range of a camera is much smaller than that of a human eye. So a scene that probably looked normal in person turns into something dramatic in photographs. (And I bet Photoshop was used to enhance that drama even more.)

The conclusion I draw from this is that bad lighting produces bad images, and that politicians are not hyper-competent when it comes to such things. The content of the speech reinforces this -- far from being an eliminationist screed, Biden's speech is yet another hand extended to moderate Republicans inviting them to leave the Trump coalition and reject fascism. (Much like Clinton's "Basket of Deplorables" speech, I expect this invitation to be ignored, but that's another matter.) Here are some representative quotes, none of which sound particularly fascist to me:

> Democrats, independents, mainstream Republicans: We must be stronger, more determined, and more committed to saving American democracy than MAGA Republicans are to — to destroying American democracy.

> So I want to say this plain and simple: There is no place for political violence in America. Period. None. Ever.

> On top of that, there are public figures — today, yesterday, and the day before — predicting and all but calling for mass violence and rioting in the streets. This is inflammatory. It’s dangerous. It’s against the rule of law. And we, the people, must say: This is not who we are. (Applause.)

> We’re a big, complicated country. But democracy endures only if we, the people, respect the guardrails of the republic. Only if we, the people, accept the results of free and fair elections. (Applause.) Only if we, the people, see politics not as total war but mediation of our differences.

> The soul of America is defined by the sacred proposition that all are created equal in the image of God. That all are entitled to be treated with decency, dignity, and respect. That all deserve justice and a shot at lives of prosperity and consequence. And that democracy — democracy must be defended, for democracy makes all these things possible.

This is bog-standard Americanism. The idea that it's some sort fascist rally speech simply doesn't hold water.

[1] https://billypenn.com/2022/09/01/biden-philadelphia-speech-i...

[2] https://www.inquirer.com/news/philadelphia-joe-biden-picture...