What Causes Toe Nails To Be Brown

Just trying to find out why my toenails are brown. Well, it's not something that'll hurt me, but I'm just curious. I didn't have brown toenails when I was born. It started turning brown when I was like around 5.I know it's not fungus because I've had it for years and it still doesn't hurt, and all my shoes are loose (1 finger width space left in the shoes or more).