The text color is specified by the rz-cell-data class in the themes. This is why setting it at td
level won't work. You can use a column template as in our demo:
<Template Context="data">
@if (data.Quantity > 20)
{
<span style='color: green'>@data.Quantity</span>
}
else
{
<span style='color: red'>@data.Quantity</span>
}
</Template>