I stick to plain white cards, except for the yellow separator and red "stop" card. Everything that goes through my head -- what to pick up at the store, cool projects to do one day, homework -- ends up an equal item on the card(s). I don't differentiate until I process the full cards.

This does require you to retain some "metadata" about each item on the list, though, like it's destination/priority. You could append each item on the list with a code for what list it will end up on (SM for someday/maybe, for example). I think this might remove bias for items. (An item on a dedicated someday/maybe card will stay a someday/maybe item, even if it's more like a regular project when you think about it hours later).