I've used TemplateField instead of BoundField which resolved my problem, it might help someone else so I will put the codes here.
<asp:TemplateField HeaderText="وضعیت">
<ItemTemplate>
<span style="background-color: <%# (Eval("ostatus").ToString() == "Accepted") ? "green;" : (Eval("ostatus").ToString() == "Denied") ? "red;" : "blue;" %>">
<%# Eval("ostatus") %>
</span>
</ItemTemplate>
</asp:TemplateField>