Fontana will throw blooms of any color it pleases from time to time. It usually opens apricot but turns pink as it ages. But, check the pot to see if there is more than one plant. Also, it might be what is called a "sport", that's when a stem of a rose blooms in a different color (not a sucker from a rootstock, your minis are own root so that's not an issue). If that stem continues to produce yellow blooms, you can try to propagate it to see if it is stable.

Propagate meaning to take a cutting from the stem with the yellow flower, root it, and if the rooting takes hold, see if the blooms from the cutting are also yellow. That's what I meant by "stable".