I have a stacked bar chart with text marks that I want coloured according to field ‘text_value’.

This is what I have now, however the top text values should be coloured black according to the color condition in the text mark encoding.

{
"data": {
    "values": [
      {"category": "a", "xval": 1, "yval": "top", "text_val": 1},
      {"category": "b", "xval": 5, "yval": "top", "text_val": 1},
      {"category": "c", "xval": 8, "yval": "top", "text_val": 1},
      {"category": "a", "xval": 3, "yval": "bottom", "text_val": 2},
      {"category": "b", "xval": 1, "yval": "bottom", "text_val": 2},
      {"category": "c", "xval": 9, "yval": "bottom", "text_val": 2}
    ]
  },
  "encoding": {
    "y": {
      "title": null,
      "field": "yval",
      "sort": "descending"
    }
  },
  "layer": [
    {
      "mark": {
        "type": "bar",
        "tooltip": true
      },
      "encoding": {
        "x": {
          "title": null,
          "field": "xval",
          "type": "quantitative",
          "aggregate": "sum"
        },
        "color": {"field": "category"},
        "order": {"field": "category"}
      }
    },
    {
      "mark": {
        "type": "text",
        "tooltip": true,
        "fontSize": 100
      },
      "encoding": {
        "x": {
          "title": null,
          "field": "xval",
          "type": "quantitative",
          "aggregate": "sum",
          "stack": "zero",
          "bandPosition": 0.5
        },
        "text": {
          "title": null,
          "field": "xval",
          "type": "quantitative",
          "aggregate": "sum"
        },
        "color": {
          "condition": {
            "test": "datum.text_val == 1",
            "value": "black"
          },
          "value": "white"
        },
        "order": {"field": "category"}
      }
    }
  ]
}

Please note: It seems the ‘text_value’ field needs to be used somewhere else, but I don’t need it anywhere except the condition. Also the aggregation must be sum, as this is a simplified example.