According to a 2008 study by the company Clairol, 75 percent of women dye their hair. Mens' hair styles are generally so short, you don't know what it would look like long. My hair looks blonde when I have it shorn short, and when it's shoulder length, it looks black under artifical light, while under natural light, it looks dark red/auburn.

Essentially, you have no idea what colour peoples hair would be naturally. None of us do just by looking around. I'd say maybe 1 in a 100 blondes I see is naturally blonde, but most of them are convincing fakes. Even in czech republic, which is just about as blonde as norway to the casual observer. They all come out of a bottle. Really, the only indicator of what people might look like naturally is men with long hair. If you're just going by visuals, you have to count out pretty much all women and any men with short hair, because it will skew your results.

The nazi regime also caused a fairly significant rise in the birth rates, and those children had a very particular set of features, in a very particular area, and they didn't disappear at the end of the war. It's a touchy subject.

What we think of as redheads, the pale freckly, orange haired sort, are mostly in ireland and northern scotland. Formerly Goidelaic speaking gaels from northern portugal that migrated in prehistory. The regions of the UK with lots of gingers are generally not areas that were colonized by anglo-frisians, iron age norse, nor french normans.

Tacitus didn't really take the time to ask around about what people called their gods, so his hot take on eye colour probably isn't any more accurate than me saying all italians are brown eyed, fat, and talk with their hands. Are plenty? Sure. Enough for that to not be racist? Not really.

You are correct that tribes of people where fair skin, blue eyes, and red hair was common became less common as those tribes became less isolated. That almost certainly would lead to those traits becoming less common. Which is why you still see lots of redheads in rural areas where there have always been lots of redheads. So yeah, that might account for a lot of it.

So, it's a little of all of the above. Modern people don't have natural hair, redheads would by nature of genes become less common, tacitus was probably what we'd consider racist in modern times, and their definition of red probably included what we'd call auburn or brunettes.

Have I met some big northumbrians with blue eyes and vast red beards? Yep. Were they the decendents of vikings? Almost certainly. Might there have been enough guys like that back in the day to make tacitus use a broad brush? Probably.

But, like all things in history, saying for sure is difficult. We learn more all the time, and some things we just don't know. Genetic analyis is becoming cheaper by the year, so we might be able to start answering questions like yours with certainty someday.