Answer

A distinction should be made between the general principle, which should be applied according to what exists in your own time, and specific Jewish laws that were established in the past (such as red clothing, etc.), where there is room for discussion. With rabbinic enactments, the accepted rule is that even if the reason no longer applies, the enactment does not lapse unless it is formally repealed by a court of equivalent authority and number (although see my article here on the site about repealing enactments). In Torah-level law, there is more room to be lenient, since the specific rules probably constitute only an application of the Torah-level law and not an enactment in themselves, and therefore it is reasonable to interpret them according to the context.