Are you trying to set visible="true" of the panel with AJAX? If this is so, then to solve your problem you need to register the CSS that the ColorPicker uses in the head of your page:
<link href="Skins/ColorPicker.css" rel="stylesheet" type="text/css" />
<link href="Skins/Default/ColorPicker.[Used_Skin].css" rel="stylesheet" type="text/css" />
In case you have a different scenario and continue to experience the same problem, please provide more information and a simple running project that reproduces the issue and we will work on it. Let me know how it goes.
the Telerik team