I made a minimal Reproduction from your example.

As you can see in there the part {..., color: ['#1890ff']} is the issue. A color property is not documented on setOption but it seems to set the color for all items in the plot. When using a theme this value is most likely overridden by the theme default values after it was initially set. Thats why its working with the "dark" theme.

If you remove the mentioned line the colors are equal for both themes:

option = {
  xAxis: {},
  yAxis: {},
  legend: {},
  series: [
    {
      type: 'scatter',
      name: 'A',
      data: [[1,2],[2,5],[7,1],[3,9],[6,8]]
    },
    {
      type: 'scatter',
      name: 'B',
      data: [[4,2],[3,5],[7,7],[4,6],[2,7]]
    },
    {
      type: 'scatter',
      name: 'C',
      data: [[4,5],[7,1],[5,3],[8,9],[6,2]]
    },
    {
      type: 'scatter',
      name: 'D',
      data: [[4,8],[7,5],[6,6],[3,2],[7,3]]
    }
  ],
  
  color: ['#1890ff'],  // <-- Remove this
};

I made a minimal Reproduction from your example.

As you can see in there the part {..., color: ['#1890ff']} is the issue. A color property is not documented on setOption but it seems to set the color for all items in the plot. When using a theme this value is most likely overridden by the theme default values after it was initially set. Thats why its working with the "dark" theme.

If you remove the mentioned line the colors are equal for both themes:

option = {
  xAxis: {},
  yAxis: {},
  legend: {},
  series: [
    {
      type: 'scatter',
      name: 'A',
      data: [[1,2],[2,5],[7,1],[3,9],[6,8]]
    },
    {
      type: 'scatter',
      name: 'B',
      data: [[4,2],[3,5],[7,7],[4,6],[2,7]]
    },
    {
      type: 'scatter',
      name: 'C',
      data: [[4,5],[7,1],[5,3],[8,9],[6,2]]
    },
    {
      type: 'scatter',
      name: 'D',
      data: [[4,8],[7,5],[6,6],[3,2],[7,3]]
    }
  ],
  
  color: ['#1890ff'],  // <-- Remove this
};

Yes, this is a reasonable explanation.

But I think the expected behavior should be the chart color when using theme, the same as not using theme .

option.color priority higher than theme

Currently the configured color option will be merged into the specified color in the theme options, instead of the whole replacement. For the color array, you only define 1 color, which is less than the number of theme color, so it finally only replaced the first color. (But I also feel that the merge strategy is a bit strange in this case)