I've been looking a lil bit on the Internet for clues how to get the best possible background in Ellen's part, but I haven't foun exactly what I'm doing different from the walkthrough. Is the white background unobtainable due to some bug?

I tried everything and I don't think it's possible to raise her psych profile any higher, but you should still get the Totem of Valor in the final scene. Although, I'm not sure you even need to use that to get the 'best' ending.

Thirding this. Can't seem to get Ellen with white background.

According to the official strategy guide there was originally an alternate ending to Ellen's scenario where the player can leave the pyramid after learning that AM's original components no longer exist instead of finding and using the Chaos Trebler. I suspect it would have worked similar to Nimdok's scenario (which has a "best" ending with a white background but also several "good" endings with a lower spiritual barometer result) But the game is bugged so the alternate endings aren't operational in Ellen's scenario, which I guess accounts for the inability to fill her spiritual barometer.

As others have said, though, it has no impact on the endgame.