The problem is that there was a problem with the minDate on the version you are using, you should update to the new version and the same thing will happen for minDate also, so this problem was fixed.
What you are talking about with the min and max date to show the dates in a differente color shouldn't be achieved using this technique, because when you are setting the minimum / maximum date you are disabling any other date outside of that interval, that's why the buttons on the calendar have that style (disabled).
If you would like to have that possibility you could always do the date checks internally and ignore the MinDate and MaxDate, you could just use two internal properties for max and min dates, and whenever a date change occurs, you can do the following checks inside the Value Changed Event
It's less clearer like this in my point of view, but it will do what you want.
sender, EventArgs e)
(MinDate.CompareTo(radDateTimePicker1.Value) > 0 || MaxDate.CompareTo(radDateTimePicker1.Value) < 0)
radDateTimePicker1.BackColor = Color.Red;
radDateTimePicker1.BackColor = Color.Transparent;
Please let me know if i can help you with something else,