iacisrmma:
Koheles 9:8 בְּכָל־עֵ֕ת יִֽהְי֥וּ בְגָדֶ֖יךָ לְבָנִ֑ים
Cute. Now let’s address the actual message of that posuk.
Start with Rashi, based on the gemora (Shabbos 153a) that explains this is about behavior, and it refers to the beauty of the expected way that we are to present ourselves. Not at all about attire.
Targum focuses directly on white referring to cleansing from sin. Several meforshim (Akeidas Yitzchok, R’ Aharon of Karlin, R’ Mendel of Kotzk) describe this as a metaphor, where one is donned with pure white clothing, and is also balancing a vessel of oil on his head. The slightest deviation from being perfectly straight and steady (referring to one’s focus on Yiras Shomayim) will lead to the oil spilling over everything and making a mess out of the clothing. Once again, the message about clothing is just a metaphor.
I return to my original question. Is there a Torah source for wearing white shirts?