Flame Bonsai stem turning brown

Hello! I'm new to growing bonsais and I'm currently worried about one of my flame bonsai. I keep it outside and it gets lots of sunlight but kept in a slightly shady area. I water it whenever the soil is dry. It's about 15 days old and the bottom of the stem is turning a brownish-green color, but the color stops once it reaches the cotyledons and the rest of the stem and all leaves are still bright green. Is this just the stem slowly becoming bark or am I doing something wrong? 

Hello there!

I can't see any picture, but sounds like what you have is a Flame Tree seedling, and well, yes it's normal for it's stem to change color as it grows.

And just so there's no confusion, there are no "bonsai seeds" or a Bonsai that is 15 days old from seed, that's a regular tree seed, it's the techniques and shapes you give to the tree that makes it a Bonsai.

You might consider choosing a more suitable species for your area, this species has huge leaves and it will take you about 15-20 years to build a real bonsai out of it, from seed.