Example Contestants are Yellow Faces (previously armless silhouettes that resemble Golf Ball) that show off how challenges are supposed to be played out, and sometimes used to show some other non-challenge related things.
In BFDI, they were green or white armless balls with no features that have a strong resemblance to Golf Ball. They were first seen in "Cycle of Life" to show off the challenge.
In BFB, instead of being objects, they are Faces, but are noticably more expressive and have more movements. They are typically yellow, but as seen in "What Do You Think of Roleplay?", they can be purple, lime or cyan.
They appear again in "Let's Raid The Warehouse", but are considerably more crude.
They made their debut appearance in "Cycle of Life".
They made a second appearance in "Insectophobe's Nightmare".
They made their return as Yellow Faces starting in "Four Goes Too Far".
They were seen again in "What Do You Think of Roleplay?", with different colored variants.