Fehling's solution turning yellow and then black? While testing for formaldehyde (an aldehyde), I added Fehling's solution to the sample and let the test tube simmer in a water bath for some time. I returned ~15 minutes later to see a black/dark grey precipitate awaiting me, which looked very metallic (Almost like Tollen's test, but darker)

I try again and this time heat the tube for only five minutes: I obtain a buff yellow coloured precipitate which, on standing, gives me the same metallic precipitate I observed earlier.

I expected that Fehling's solution will give a red precipitate of $\ce{Cu2O}$ but it looks like the formaldehyde seems to be oxidizing it all the way down to metallic copper. Also, what is the reason for a yellow precipitate to form rather than a red one?

I could not take any pictures: We're not allowed to carry phones/cameras into the lab.
In the reaction with aldehydes, Fehling's solution produces $\ce{Cu2O}$ as an orange or yellow precipitate. I don't know why the color is not always the same. The oxidation of an aldehyde produces an acid. And usually an acid cannot be more oxidized. There is one exception : methanal $\ce{HCHO}$. Because it can be oxidized in formic acid $\ce{HCOOH}$, and then into $\ce{CO2}$ . That is why Fehling's reagent produce metallic copper $\ce{Cu}$, $\ce{CO2}$ and no acid when reacting with $\ce{HCHO}$.