Web Forms :: Find Hyperlink Control In Master Page From Content Page?

I have hyperlink in page

<asp:HyperLink ID="HyperLink1" runat="server" class="lblMenuMessage" NavigateUrl="~/Admin/صندوق-پیام.aspx">صندوق پیام</asp:HyperLink>

and css

.lblMenuMessage
{
float:right;
text-align:right;
width:150px;
margin:0 12px 0 0;
color:white;
text-decoration:none;
}

and I want change it's color in behind code so I wrote:

(this.Master.FindControl("HyperLink1") as HtmlGenericControl).Style["color"] = "black";

but below error happen:

Object reference not set to an instance of an object.

Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code. Exception Details: System.NullReferenceException: Object reference not set to an instance of an object.Source Error: 

Line 13:
Line 14: (this.Master.FindControl("ADMenuMessage") as HtmlGenericControl).Style["background-image"] = Page.ResolveUrl("~/Image/ADactivmenu.png");
Line 15: (this.Master.FindControl("HyperLink1") as HtmlGenericControl).Style["color"] = "black";
Line 16: }
Line 17: }

how I can change hyperlink textcolor in behind code?