@{
   int counter=0;
 }
 @foreach (var item in Model)
 { 
   counter++;
   <div class="@(counter<=3 ? "classRed":"classBlue")">
       <img src="@item.Blog.Image.img_path" alt="Not Found" />
       //other markup also here

   </div>  
    if (counter == 6)
    {
        counter = 0;
    }

 }

classRed 和 classBlue 是CSS类

Where classRed and classBlue are the CSS classes