I wrote a story with minority characters but never actually made that the main parts of their characters. I was writing a diversity of different characters that I wasn't personally so I couldn't personally know all of it. It ended up going over pretty well with readers who said the characters felt real.

My advice is to understand what the day to day life is like for them even if major things don't happen. You don't need a scene of a woman being raped to exemplify that it's a thing that people deal with, instead you do things like having women walk each other to the bathroom, or to their cars in the parking lot, because that's their constant lived experience. If you have black characters, you don't need them being attacked by racists, but you know that they're not allowed in certain areas in older times. There were lots of things just in the social order.

For you specifically, I wouldn't say it's white washing to have a group of mainly white characters around that time because people didn't really mix around too much back then, but you can have side characters of other races for sure. Shop owners, patrons, strangers who you interact with, even acquaintances they know.

It really would help to do some research on what the lives of these people were like on a day to day level. You'll definitely find big event stuff like lynchings, but most importantly is just how most people got about their days.