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The colour disappears that quickly because it is dusting. When a pale colour is cause by dusting it virtually always it disappears when the specimen is immerged in alcohol. You see the same thing happening with, for example, Dolichopodidae (dull grey-green Medetera instantly turn metallic green), Chamaemyiidae (distinctive silvery grey Leucopis suddenly are non-descript dark little flies) and numerous others. Since many dust patterns are in someway distinctive (helping identification) many entomologists are not to keen on material in alcohol. However, with a trained eye and good lighting you can still see the dust patterns. The colour of ducting, now that can be more problematic... ;)