Honestly, if you must dye your hair, go for light brown.
After a baby, your hair thins to an extent (mine did :'( ) so I would steer clear of anything that you have to bleach it for, maybe if you want a lighter look, go for highlights. You might want to go for a look that will last, and I'm speaking from experience, cause I hated when I got a purple dye after my baby. It looked so shitty cause it was starting to fade, and yet I couldn't do much since I was trying to save money, and I was too busy with baby to get a touch-up the week before. Black is okay, but I don't think it does as much justice for your skin tone as the light brown does. Just my two cents. But you go for what you like, mommy! :)