I planted an apple seed a few weeks ago and it sprouted and was doing great but just recently the stem suddenly turned reddish brown one night? The leaves look fine but I'm a bit concerned about the stem. I'm using potting mix and a little bit of organic compost for the soil.

Here is a picture of it:

It appears to be OK. The stem would be expected to turn colors (brown or perhaps reddish) as it starts developing woody structures.