Could you please elaborate how have you set the color change in your project. Are you adding those particular days in SpecialDays collection and change the collection's color in OnRender?
I suggest that the problem could be caused by the OnMouseOver style (Hoover style), which overrides the custom style you have set. Could you please try to increase the weight of the custom style by using "important" in your CSS class(is such is set).
the Telerik team