Really, you need to always look at where your layout takes place and when it does, to determine the makeup of your scale population. My layout takes place in the extremely rural Northeast Tennessee during WW2. That means:
An almost exclusively white population. Blacks simply didn't live up in those 'hollers' back then, and as far as I know, probably still don't.
Very few able-bodied men of any race that aren't in uniform or working for the railroad (which was an easy exemption for the draft at the time). So most of my figures are of men too young or old to be in the service, able-bodied men either in uniform or RR employees. The rest are women of varying age (and many of them older women as there were rayon mills nearby where most younger women would be working).
But this would be totally different if the layout took place, say, where and when I grew up in the early 70s/80s in North Florida. A massive black population would need to be well-represented if you wanted such a layout to be accurate. Where I live now, in the Pacific Northwest, there is a very light black population. We have a large Asian population, though, but they live in clusters for the most part. If you went to the small town I live in now, you'd think only white people (and the Native Americans from the various local reservations) live here.
So, every place is different. Just look at the demographics of your layout's time/place and go with what's the most likely. Frankly, I find that kind of research fun to do, as I always want to know what's historically right.
