What are the standard ways to return API HTTP Status Code for Warning

No, it is perfectly fine to use your own implementation, with Ok(result), as a developer I don't think it is not acceptable or a violation.

Yes, we should try to follow the Rest principles, which say that respect the HTTP protocol means sending the appropriate status code with the response. 

There are many HTTP status codes like 303 that the result of a post request matches with any existing representation.

422 unprocessable entity etc.

but 200 is also a status code at the end, so is fine as long as the response clearly describes the problem. 

if you are using IHttpActionResult instead of HttpResponseMessage then we already have helper methods for necessary status codes as

• BadRequest()
• Conflict()
• Created()
• InternalServerError()
• Redirect()
• Unauthorized()

public IHttpActionResult Get()  
{  
   var empList= db.Employees.ToList();  
    return Ok(empList);  
}  

public IHttpActionResult Get(int id)  
{  
    var employee = FetchFromDB().FirstOrDefault(e => e.Id == id);  
    if (employee == null)  
    {  
        //return NotFound();  
        return Content(HttpStatusCode.NotFound, "employee not found");  
    }  

    return Ok(employee);  
}