Threshold value for color conditioning to be taken from a variable/field rather than giving direct number

[Pranusha119 (Pranusha)]

     "color": {
       "condition": {
         "test": {"field": "PercentOfTotal", "gt": 70},
         "value": "#B22222"
       },
       "value": "#006400"
     }

By this way I am able to do color conditioning

     "color": {
        "condition": {
          "test": {"field": "PercentOfTotal", "lt": datum.a_max},
          "value": "#006400"
        },
        "value": "#B22222"
      }
      }

When I am taking conditioning value from a variable(a_max field created using max aggregation), it's giving default color.

Is there any way where I can take threshold value for color conditioning from a variable/field rather than giving direct number.