What color hairdye do you use on brown hair to get natural blonde?

Any colours you want but if you are going extreme it is best to go to a salon.

light brown hair

Nothing.. If you use lemon juice it is horrible for your hair because it is very acidic.

Blonde is a recessive gene and Brown is dominant. Therefore, your child's hair will have an extremely high probablity of being brown.

Due to dominant vs. recessive genes the most likely answer would be brown, but if the father has an extended blonde family the child may be blonde.