Reproduction steps:
<telerik:RadEditor runat="server" ID="RadEditor1"> <Content> <span style="color: #548dd4;">asdas</span>this is sample text </Content></telerik:RadEditor><span style="color: #548dd4;">asdas</span>this is sample text<span style="color: rgb(84, 141, 212);">asdas</span>this is sample text
<script> Telerik.Web.UI.Editor.ConvertToXhtmlFilter.prototype._convertRgbToHex = function (text) { if ($telerik.isFirefox || $telerik.isSafari || $telerik.isIE9Mode || Telerik.Web.Browser.chrome || Telerik.Web.Browser.edge) { text = text.replace(/rgb *\( *\d+ *, *\d+ *, *\d+ *\)/gi, function (match, offset, fullText) { return Telerik.Web.UI.Editor.Utils._rgbToHex(match); }); } return text; }</script>