system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why did my hair change from blonde as a child to brunette in my teens?

Why did my eyes go from crystal blue to green/blue? Hair from bright blonde to light/medium brunette? I'm male, it it matters.
gpt
Hair and eye color darkens over time because the respective genes get turned on during your growth.

For people to get the traits encoded in their genes, the genes need to be turned on. You can think of a gene as a recipe. A recipe in a cookbook does you no good until you open the book, get the ingredients and follow the instructions

The same is true with our genes. In our analogy, the gene is the recipe and the cookbook is the chromosome (a large collection of recipes). The gene needs to be "read" by the cell in order for it to do what it should. An unread brown or black hair recipe will give you blonde hair.