Dark leaves

Hola amigo, bonito bebe, parece ser genetica o frio pero parece muy sana, parece mas genetica, feliz cultivo 💚🍀🌱🍀💚

They are not leaves, they are cotyledons and basically play no part in the growth of the plant. Their main "job" is to protect the growing tip as it pushes through the soil. The first true leaves will emerge from the center area once the cotyledons fold back and open. That they are of a purplish color is of no consequence and is quite common. They will dry up, die and fall off (usually) within the first 2-3 weeks................all this is normal.

genetics or cold temps

At this stage dark purple cotyledons are most often just genetics (anthocyanin), and many fastflower and purple strains do it. It's usually harmless and often fades as true leaves come in and green up. Other causes to rule out: cold roots or air (keep soil around 21-24°C), a too-wet medium, and light that's too close or too intense, so keep the seedling lamp at a gentle 200-300 PPFD. Don't feed anything yet and water only around the seedling. If the true leaves also come in dark or the stem turns purple and growth stalls, check the soil pH (6.0-6.5) and temps