A side effect of "Persistent Colors" is that you may get colors duplicated on a chart, even with less than 18 values shown on a chart. When Persistent Colors is checked, QV pre-assigns colors to all values in the field in load order. If you have more than 18 values in the field, the assignment just rotates through the same 18 colors again. So value 19 will have the same color as value 1.
thank you for your clarification. Because in the field there are more than 10.000 values, I think there are many case to find same colour for different values. Is there any possibilities to set more than 18 colours or the only solution is to uncheck "Persistent colour"?