Good question; I don't think it's color related (like believing red fish -- e.g. Arowana & Flowerhorn will bring good luck & prosperity, which is pretty much based on Chinese culture/mythology).
In the case of platinums/albinos my guess is it's mostly due to just wanting something different, and they may consider many 'normal' fish a bit blase because for a long time they have had much easier/cheaper access to them overall compared to their fish keeping counterparts in Western countries. So for some maybe it's kind of a been there done that kind of thing, and always looking for something new/different. Platinums & albinos also usually cost more, so it could be kind of a status symbol thing for some peeps.