Leviticus 13:47

And a garment in which is the plague of leprosy, whether a garment of wool or a garment of linen,

AND WHEN THE PLAGUE OF LEPROSY IS IN A GARMENT. This is not in the natural order of things, nor does it ever happen in the world [outside Israel], and similarly leprosy of houses [is not a natural phenomenon]. But when Israel is wholly devoted to G-d, then His spirit is upon them always, to maintain their bodies, clothes and houses in a good appearance. Thus as soon as one of them commits a sin or transgression, a deformity appears in his flesh, or on his garment, or in his house, revealing that G-d has turned aside from him. It is for this reason that Scripture states, And I shall put the plague of leprosy in a house of the Land of your possession,, 14:34. meaning that it is G-d’s punishment upon that house. Thus [the law of leprosy of houses] applies only in the Land which is the inheritance of the Eternal even as He said, When ye are come into the land of Canaan, which I give to you for a possession., 14:34. Now the reason [why this law does not apply outside the Land of Israel] is not because it is a duty which attaches to the ground, but the reason is because this matter [of Divine indication of sins] occurs only in the Chosen Land, wherein the Glorious Name dwells. And in the Torath Kohanim the Sages further interpreted 5:3. that a house does not contract impurity until after the conquest and division [of the Land by Israel], and until after each and every individual clearly knows his portion. The reason for this law is that only then do they have the ease of mind to know the Eternal, and the Divine Glory dwells among them. I think similarly with reference to the law of leprosy in garments, that it applies only in the Land [of Israel], it being unnecessary to exclude [the application of this law] in places outside the Land, since they never occur there. For this reason also the law of leprosy in garments applies only to white garments, not to colored ones, because the color might perhaps have extracted this unclean phenomenon [i.e., the leprosy] in that place [in the garment] in a natural way, and it will then not be regarded as a finger of G-d. Therefore garments colored by Heaven can contract impurity, according to the words of Rabbi Shimon. And by way of the simple meaning of Scripture, the reason why it repeats in every verse the expression “the garment, or the skin, or the wrap and the woof,” is because the matter is miraculous. Our Rabbis have interpretations for them [i.e., these repetitions], and all of them are found in the Torath Kohanim.

והבגד כי יהיה בו נגע צרעת, when it is quite clear that this is not the kind of stain familiar to science and responsive to removing by chemical detergents. Discolouration of a garment in colours other than the ones that are work related due to the vocation of the wearer, are usually caused deliberately by the owner. What occurred here is due to some sin the owner was guilty of.
At any rate, our tradition has taught that the only discolouration of garments or cloths that are considered from a halachic point of view are garments which themselves are white and not coloured. Any coloured garment is not subject to this legislation at all, is not within the parameter of what is called נגע צרעת. The phenomenon of white garments or exterior walls of houses which are normally whitewashed breaking out in different colours is by itself a warning to the owner to examine his lifestyle and what he might have done wrong to rate such a warning.
Our sages in Kidushin 20 tell about how seriously a violation of a rule which is only peripheral to the Sh’mittah legislation is viewed. [the basic rules of sh’mittah relate to working the soil during that year. What I described here as “peripheral,” i.e. in Talmudic parlance as אבקה של שמטה, is the dealing in produce grown in violation of the sh’mittah laws. Ed.] The Talmud explains that violating the “peripheral” aspects of that legislation my result in the guilty party becoming impoverished. The process is gradual so that the guilty party had ample opportunity to ask himself why G’d had singled him out for this kind of punishment.
The same is true of the נגע צרעת legislation. Proof of the fact that this affliction is due to a well meaning Creator, One Who is concerned with the welfare of His Jewish subjects, is the fact that if the symptoms we know as נגע צרעת surface on the garments of gentile or houses of gentiles, these are not declared as ritually impure. Seeing that the gentiles as a rule do not have life in the hereafter to look forward to, G’d is not at such pains to warn them not to risk losing something they doe not to look forward to in any event. When a Jew is in danger of losing his claim to eternal life this is a far more serious matter, and G’d goes out of His way to afford the person potentially endangering his claim to eternal life an opportunity to repent and change his lifestyle before it is too late. In referring to this concept, Solomon in Proverbs 10,25 describes the צדיק, the Just, as the …