Hello,

I had the same issue and I didn't want extra postback + my "colouring" was depending on server-side logic, which couldn't be migrated to JS.

Perhaps my solution will help someone as yours helped me. In short, I used CustomAJAXResponse.AdditionalProperties.

So let's assume we have a bunch of cells which style we want to change. 

1) We form by some logic on the server side the array of these cells. For example,  GridRecordItem[] cells.

2) We add "addresses" of these cells to customAjaxResponse. There are different ways to do that - using indexes (which I wouldn't recommend because if something in your code sorts the values it can cause problems) or idpairs for rows, I just used row.DataKey in the following code:

int i = 0;
foreach (GridRecordItem cell in cells)
{
    this.WebDataGrid1.CustomAJAXResponse.AdditionalProperties.Add("rowKey" + i.ToString(),
                                                                                                    cell.Row.DataKey);
    this.WebDataGrid1.CustomAJAXResponse.AdditionalProperties.Add("columnKey" + i.ToString(),
                                                                                                    cell.Column.Key);
i++;
}
this.WebDataGrid1.CustomAJAXResponse.AdditionalProperties.Add("Count", i);

Please note, that for this you must have columnKeys assigned.

3) In AJAX postback handler on the client side we set our styles.

function WebDataGrid1_Grid_AJAXResponse(sender, e) {
    var grid = $find('<%= this.WebDataGrid1.ClientID %>');
    var properties = e.get_customResponseObject().AdditionalProperties;
    for (var i = 0; i < properties.Count; i++) {
        var rowKey = properties["rowKey" + i];
        var columnKey = properties["columnKey" + i];
        var row = grid.get_rows().get_rowFromKey(rowKey);
        if (row != null) {
            var cell = row.get_cellByColumnKey(columnKey);
            if (cell != null) {
               cell.get_element().className = "YourCSSName"; 
            }

        }
    }
}