Let's compare the birthday bass and birthday tenor. They are so alike.
This is what he looks like.
The Birthday Bass is a fanmade birthday sing a ma jig. He is lime green, has the same ears, hands, and feet, like all birthday sing a ma jigs, has circle eyes just the same as the birthday tenor, has a voice that is a mix of the new red one and the Skeleton, has a light blue nose and a mouth the same color as the lime green's nose. He has a blue shirt with orange polkadots and happy birthday in green lettering. He signs on by saying "Happy Birthday!", signs off by saying "Bye Bye, Birthday!" just like the Birthday Soprano and enters song mode by saying "Birthday Song!". Once again he sings "Happy Birthday to You" like all birthday Sing-a-ma-jigs.