You might be able to calculate a TOPN rank measure (1 to 10) and put this in your Legend field instead of the current field you have in there. Then the number 1 to 10 should never change colour, but unfortunately you would the item name would be replaced by the number 1 to 10 on your legend.
In that case, you could put a table or matrix (or slicer) nearby on the same report page that listed both the item rank and the item name. e.g.
1. Green Eggs
2. Ham
3. Cheese
4. Toast
..
10. Peanut Butter