If I change the colors in the default or any other color style, will those changes remain after an update? If all I want to do is change colors, must I add a new theme to do that?
All option changes will stay the same after update. The color options as well. You can also copy /classic/ theme files to /themes/your-theme/wpforo/ folder to do more customization. You can find more information in Dashboard > Forums > Themes admin page.