The workaround is to convert the color names to HEX, for example through the following HexConverter() method:
ASPX:

        <telerik:RadClientExportManager runat="server" ID="RadClientExportManager1">
        </telerik:RadClientExportManager>
        <telerik:RadButton ID="RadButton1" runat="server" OnClientClicked="exportRadHtmlChart" Text="Export RadHtmlChart to PDF" AutoPostBack="false" UseSubmitBehavior="false"></telerik:RadButton>
        <telerik:RadHtmlChart ID="RadHtmlChart1" runat="server" Width="600" Height="400">
            <PlotArea>
                <Series>
                    <telerik:ColumnSeries Name="Series 1" DataFieldY="myValues" ColorField="Color">
                    </telerik:ColumnSeries>
                </Series>
            </PlotArea>
        </telerik:RadHtmlChart>
        <script>
            var $ = $telerik.$;
            function exportRadHtmlChart() {
                $find('<%=RadClientExportManager1.ClientID%>').exportPDF($(".RadHtmlChart"));
            }
        </script>

C#:

    protected void Page_Load(object sender, EventArgs e)
    {
        DataTable dt = GetData();

        for (int i = 0; i < dt.Rows.Count; i++)
        {
            //Not all Color Names can be exported
            // dt.Rows[i]["Color"] = ColorTranslator.ToHtml(GetColor(i));

            //Convert colors to HEX works properly
            dt.Rows[i]["Color"] = HexConverter(GetColor(i));
        }
        RadHtmlChart1.DataSource = dt;
        RadHtmlChart1.DataBind();
    }

    public static DataTable GetData()
    {
        DataTable dt = new DataTable("ChargeData");
        dt.Columns.Add("myValues", typeof(int));
        dt.Columns.Add("Color", typeof(string));


        dt.Rows.Add(11, "");
        dt.Rows.Add(12, "");
        dt.Rows.Add(13, "");
        dt.Rows.Add(14, "");
        dt.Rows.Add(15, "");
        dt.Rows.Add(16, "");
        dt.Rows.Add(17, "");
        dt.Rows.Add(18, "");
        dt.Rows.Add(19, "");
        dt.Rows.Add(20, "");
        return dt;
    }

    public static String HexConverter(System.Drawing.Color c)
    {
        return "#" + c.R.ToString("X2") + c.G.ToString("X2") + c.B.ToString("X2");
    }

    public static Color GetColor(int index)
    {
        return colors[index % colors.Length];
    }

    public static readonly Color[] colors =
    {
        Color.Orange,
        Color.Violet,
        Color.MediumSeaGreen,
        Color.HotPink,
        Color.Black,
        Color.DarkGoldenrod,
        Color.DarkMagenta,
        Color.Chocolate,
        Color.DarkOliveGreen,
        Color.DarkTurquoise
    };