The current version of the RadHtmlChart provides exactly 5 colors for the series and as you mentioned the way to use other s is to set the color of the corresponding series items. We are currently working on adding additional 5 colors and they might be introduced in one of our next releases.
There is also an item in our public issue tracker
where you can vote for this feature and as more and more clients do this we will raise the priority of this task and provide it to you as soon as possible.
The following resources can help you in using the desired colors for your charts:
the Telerik team
If you want to get updates on new releases, tips and tricks and sneak peeks at our product labs directly from the developers working on the RadControls for ASP.NET AJAX, subscribe to their blog feed