Thank you for the writing back.
You cannot add this RadDropDownColorPicker at design-time. You should add it via code into Ribbonbargroup's Items:
I hope this helps.
All the best,
the Telerik team
Do you want to have your say when we set our development plans? Do you want to know when a feature you care about is added or when a bug fixed? Explore the Telerik Public Issue Tracking
system and vote to affect the priority of the items