The Rambam says that the tzitzis strings must be the same color as the garment they're attached to (with exception to the techeiles string which is always blue). (Source?)
All of the tallisos these days are predominantly white, and so the strings are also white. In the light of the Rambam's ruling, though, what would be the halacha on strings that became discolored over time or through some other accident?
For instance, Chaim Yankel ties white strings onto his white garment, and over time the strings become yellowed but the garment stays white. Or, someone accidently spills red paint and gets some on his tzitzis. Are the tzitzis still valid, even though they aren't the same color anymore?
Obviously not everyone agrees with the idea that the tzitzis should be the same color as the garment they're attached to, but this is a hypothetical idea following the Rambam's view.