Since your background coloring will NOT depend on the even/odd index information, you should simply use another approach for the styling.

What I would do personally is use CSS classes to identify the initial even/odd rows and when a row get extended give its children the same class.

You didnt share the 'expand' code so I do not really know how you could do that but something like that:

<tr
  key={i}
  className={`Itable-row-default table-row-${i} ${i%2 === 0 ? 'table-white' : 'table-grey'}` }
>
  <td colspan={columnData.length + 1}>
    {dataExpand[i]}
  </td>
</tr>

And the associated css

tr {
  &.table-gray {
    background: gray;
  }
  &.table-white {
    background: white;
  }

}

Depending on the 'onExpandHandler' method & the format of your initial model this might need some adaptations though :)

(oh and you should favor screenshots for images :p)