Your original plant was probably a cross between an orange clivia and a yellow one. Yellow clivias have a gene that doesn't produce the pigment that makes clivias orange. A clivia only needs one of those genes to have orange flowers, so a cross between an orange with two orange genes and a yellow with two yellow genes will produce 100% orange clivias with one orange and one yellow gene. If you cross those plants, half the offspring should have two yellow genes and will have yellow flowers. That's classic Mendelian genetics.

Clivia genetics are actually a little more complicated than that, with several different genes contributing to flower color. There are different kinds of yellow clivias that have different yellow genes, called group one and group two yellows. If you cross a group one yellow with a group two yellow, you will get an orange clivia. There also are yellows that arent in either group. There hasn't been a lot of scientific study of clivia genetics, so the issue of color inheritance is still a bit of a mystery.

Yours could have unintentionally cross pollinated as well, especially if they are outdoors :)

Or it could be hetrozygous as Ohiofem said.