I created a report in Zendesk Explore using the metric AVG (Unsolved tickets age (days). When I try to change the color of the cells according to the metric's value as explained in this recipe, the background color doesn't change.
The issue comes from the fact the title of the metric contains the parentheses icon. To solve this issue, duplicate the metric and remove the parentheses out of the new name.
For information on how to duplicate a metric, see the article: Editing metrics and attributes.