Actually, it's simple genetic (about hair ofc)
Dark hair genes are dominant and blond are recessive. To have a child with blond hair the parents must both have a recessive blond gene and put in the recessive blond gene each for the child to have blond hair.
Soo, yeah lil Prince can be 100% blonde
Also, the hair is usually lighter when younger. Some genes that make the hair darker are switched on when we age (most well known is puberty). Although, if I am not mistaken, there are exceptions for some with pure black hair with no lighter genes mixed in to it (they have black from the beginning) and for pure redheads (they don't get darker). Also interesting if genes from one parent has red hair (most recessive colour) and the other platinum blond or very similar blond (second most recessive) - only then you get a platinum or milky blond who doesn't darken with age, because there are no other "colours" to awaken with time.
even if i read the spoiler, i want to explain that it’s common for babies to be born with the lighter blond and when they grown up their hair become darker, that’s can happen with eyes too.
AH WHERE DO YOU READ THE SPOILER PPL