What customizations do you wish to make to the tooltip when RadColorPicker ShowIcon="true"? This appears to be somewhat limited.
It is possible to modify the text for the tooltip that appears when the mouse hovers over the icon, This is controlled by the PickColorText
The /Localization online demo at http://demos.telerik.com/aspnet-ajax/colorpicker/examples/localization/defaultcs.aspx
demonstrates modifying these properties. Each of the localization strings can be modified at runtime using the Localization property. This saves the need to create a new localization file to change a single string. See the Localization
Hope this helps.