Thank you for contacting the Telerik's support team.
The reason for this behavior is because the RadHtmlChart uses 5 predefined colors for the series, i.e. if you use more than 5 series in a single chart, their colors will start to repeat themselves. However, we have worked on adding 5 more colors for each skin and this improvement will be available in our next Service Pack. If you are too eager to get them, you can find the new colors in our Latest Internal Build.
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 the blog feed