In Pikmin 1, candypop buds are flowers. If you throw a pikmin into a candypop bud, they turn into the pikmin color that is the color of the flower (Red, Yellow, Blue)
In Pikmin 2, they are a lot like in Pikmin 1, but the colored ones die after 5 pikmin are thrown into them (Red, Yellow, Blue, Purple, White). They are the only way to obtain purple or white Pikmin. Also in Pikmin 2, there are Queen Candypop Buds: If one pikmin is thrown in 8 red, yellow, or blue Pikmin (depending on when you throw them in) come out.