I'm saving up to buy the elf force color, but it says it's sold out. When do they restock?

i guess after a long while
or they state date

Some item will never be restocked, some will have 'limited restock' and others are re-stocked every month or so.

They'll more than likely be restocked when the next promos are made available.

You could always look in the marketplace, Tori-Market and Tori-Exchange to see if anyone's selling.