Generating a table of differences between two dictionaries

You should separate the logic from your representation and first collect those differences in a separate list. Also, what are the types of your dictionaries? All those ToString()s make it really noisy and are likely not necessary. Furthermore, use a foreach loop to iterate through your key list, this will also automatically get rid of all those keyList[i] accesses. And finally, instead of appending to a string directly, use a StringBuilder to make it more efficient:

// Use a separate type to collect your diffs
class DiffEntry
{
    public string ColumnName { get; set; }
    public string OldValue { get; set; }
    public string NewValue { get; set; }
}

// collect all diffs
List<DiffEntry> diffs = new List<DiffEntry>();
foreach (string key in keyList)
{
    if (oldDic.ContainsKey(key))
    {
        if (newDic[key] != oldDic[key])
        {
            diffs.Add(new DiffEntry()
            {
                ColumnName = colNames[key],
                OldValue = oldDic[key],
                newValue = newDic[key]
            });
        }
    }
    else
    {
        diffs.Add(new DiffEntry()
        {
            ColumnName = newDic,
            NewValue = newDic[key]
        });
    }
}

// generate the actual table output
StringBuilder tableContent = new StringBuilder();
foreach (DiffEntry diff in diffs)
{
    tableContent.Append("<tr style='border: 1px solid black;'>");
    tableContent.Append("<td style='border: 1px solid black;'>");
    tableContent.Append(diff.ColumnName);
    tableContent.Append("</td><td style='border: 1px solid black;'>");
    tableContent.Append(diff.OldValue);
    tableContent.Append("</td><td style='border: 1px solid black;'>");
    tableContent.Append(diff.NewValue);
    tableContent.Append("</td></tr>");
}
table += tableContent.ToString();

How about a generic solution using LINQ?

private string GetDiffTableRows<TKey, TValue>(IDictionary<TKey, TValue> oldValues, IDictionary<TKey, TValue> newValues)
{
    TValue oldValue, newValue;
    var diffs = oldValues.Keys.Concat(newValues.Keys).Distinct().Select(k => new
    {
        Key = k,
        OldHasKey = oldValues.TryGetValue(k, out oldValue),
        NewHasKey = newValues.TryGetValue(k, out newValue),
        OldValue = oldValue,
        NewValue = newValue
    }).Where(d => d.OldHasKey != d.NewHasKey || !Equals(d.OldValue, d.NewValue));

    var tableRows = diffs.Select(d => string.Format(DiffTableRowFormatString, d.Key, d.OldValue, d.NewValue));
    return string.Join(string.Empty, tableRows);
}

private const string DiffTableRowFormatString =
    "<tr style='border: 1px solid black;'>" +
        "<td style='border: 1px solid black;'>{0}</td>" +
        "<td style='border: 1px solid black;'>{1}</td>" +
        "<td style='border: 1px solid black;'>{2}</td>" +
    "</tr>";