That's indeed a very nasty bug you found there. I can't honestly say that I completely understand how it's happening, but never the less I've logged it for further investigation.
By the way, if you do not need the #DEF7CE colour exactly, you could achieve something similar with the shift colours tab and use the hue and saturation slides.
For that bug report, I have awarded you some Telerik points, which you can see updated in your account profile.
All the best,
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