We found some more information. 

 

If we click on the actual toolbar button itself, the colors are not applied (blue arrows).

 

But if we click on the little "down" arrow beside the button and select a color from the pallete, then the color is applied (black arrows).

 

 

 

As noted before, this is only an issue if the readonly property was set and if a telerik tab strip control is used (we also found the issue on one of our other projects which wasn't noticed before)