“A white shirt is no more or less frum than any other colour.”
If I may be blunt and clear without knocking anyone:
Would you agree that the standard of “Frum” at it’s highest humanly possible perfection would be the conduct of Gedolei Yisroel, the greatest among our Rosh Yeshivos and Poskim?
Do these Gedolim wear shirts that are white or shirts of other colors?
Case closed! So yes, a white shirt is dressing the way the frum Godol dresses (and in contrast, a colored shirt is what the frum Godol would never wear),and to repeat, the standard of Frum is set by the conduct of the Godol.
(BTW these Gedolim that only wear white shirts do not c”v conform to “our” (or anyone else’s) standards, rather, they are the ones that set the standards and it is we that try our best to emulate them).