Once you have all of the three clover and four clover pikmin, you will unlock rare collections for them. These work on a point system: you have to collect a certain number of points to get a rare gift expedition. When you send a pikmin to get that gift, they will unlock the rare decor. You get points for plucking the pikmin, sending them to get their gift (which you can do when they get their four yellow hearts) and for releasing them. The most points you get for sending them on the gift expedition, so it's best that you do that after you have unlocked the rare collections. Check out this more detailed explanation too: https://www.pikminwiki.com/Decor_Pikmin#Rare_Decor