My guess - iron electrodes
I looked up FeCO3
and found that it was, in fact, a yellowish hue; but I was actually using copper electrodes. However, I don't completely doubt these electrodes had something to do with the reaction.
Binding from the graphite often gives a yellow tint as well.
I'm afraid I didn't have any carbon in my experiment, even for the electrodes. I've had a shortage in zinc-carbon batteries, so I had to just use plain copper. Once, when I did use carbon electrodes, I had a CuCO3
precipitate developing from the connection between the graphite and copper wire. Only, it was a light blue rather than yellow, and settled at the bottom of the container.