Are you using ASP.NET Treeview? You didn't include the HTML control things in your question. If ASP.NET Treeview then see this
.clsWhite { color: White; }
<asp:treeview runat="server" xmlns:asp="#unknown">
<nodestyle cssclass="clsWhite" />
<rootnodestyle cssclass="clsWhite" />
<parentnodestyle cssclass="clsWhite" />
<leafnodestyle cssclass="clsWhite" />
<hovernodestyle cssclass="clsWhite" />
<selectednodestyle cssclass="clsWhite" />
</asp:treeview>