I don't know if hair color or physical appearance should be primary selection criteria. If they try to match physical appearance, hair color, or anything else for that matter, they will be compromising on the thing they need the most, the ability to carry the role. And who knows? a recast sometimes results in a better actor for the charter.
For example, when "Last Man Standing" was picked up by Fox after ABC dumped it, they had to recast the middle daughter, as the original actress was otherwise committed, and their choice (in my opinion) was better in the role than the original actress who had the part for 6 seasons. The oldest daughter, Kristen was also recast after season one to no ill effect.
One thing I'm sure of though. They need to ignore fan input on this. Everybody & their mother will be pushing their favorite choices based on their own preferences and agendas. What happens to anything created by committee?