You can't be sure, hair colour is determined by DNA - who gets what is totally unknown. Although there are some factors that make it more likely to get one trait or another. For example brown eyes are the more dominant trait so a blue eyed parent and a brown eyed parent will most likely have a brown eyes child.
I have blue eyes and my husband has brown eyes and my baby has blue eyes however so even the dominant/recessive gene is only a statistical probability not a 100% fact. -Edit- Granted there are statistical anomalies. In almost all cases the dominant/recessive gene concept is true, normally if it does not it is because of a mutation has occurred in the genome. In the previous answers case, every person carries 2 genes (more for some other genetic traits, 2 for the most basic). What ever gene is more dominant is the one that controls the outcome. It is more likely that the husband carried both a blue and a brown eye gene, leaving them a 50/50% probability that their children would have blue eyes.