Modify data in one column based on data value in another column or columns

Need a little help - very new to Datatables, and I'm not a programmer..

I want to modify the style (color) of the data in one column based on the value of the data in another column of the same row.

A simple example:

<table>
  <thead>
    <tr>
      <th>Foo</th>
      <th>Bar</th>
    </tr>
  </thead>
  <tbody>
    <tr>
      <td>.21</td> <!-- if this Foo number is less than .25... -->
      <td>34</td>  <!-- then this Bar number gets wrapped in a span -->
    </tr>
  </thead>
</table>

I understand that I want to use mRender, but I don't quite get how to put it all together. I've successfully added a test span based on the following example I found elsewhere in the forums:

            "aoColumnDefs": [
                {   "aTargets": [ 1 ],
                    "mData": "0",
                    "mRender": function ( data, type, full ) {
                      return '<span class="test">'+data+'</span>';
                    }
                }

but I don't understand what mData: "0" is referring to in this case, and how to get the data from column [0] and evaluate it, before returning the wrapped data for column [1]

can somebody give me a nudge in the right direction?

Thanks!