Are you streaming or using a device like a computer or console? If you are streaming the content needs to be in HDR or it will look weird like that. I think Hulu is the only service that doesn’t support it currently.
Since you have an 8k it’s probably pretty new and should detect that though.
Have you looked up a settings guide for your TV? Generally when I get a new TV I look up a guide and need to tweak every advanced setting before it looks great. People spend hours on that stuff. After doing that I definitely notice a difference