Change text colour only on certain characters within a particular dgv cell

If a cell value contains "1" (among other numbers), I would like to know:

How can I change the colour of that character within that particular dgv cell (or string)?

If a certain condition is met, e.g. a cell have a string like "1,2,3" and contains "1", then I know how to change colour on the whole row where this occures.

What I don't know is how to only change the text colour for the cell, where the condition is met. 
But I want to go further than that, and only change particular characters in the string, and only those containing "1".

How can I acheive that, if possible?

if (dgw != null)
{
   foreach (DataGridViewRow row in dgw.Rows)
   {
      //if new row skip, otherwise null reference exception
      if (row.IsNewRow)
      return;
      foreach (DataGridViewCell cell in row.Cells)
      {
         if (cell.ColumnIndex == 3) //Particular column index
         {
            if (cell.Value.ToString().Contains("1"))
            {
             row.DefaultCellStyle.ForeColor = System.Drawing.Color.Red;
 //INSTEAD CHANGE TEXT COLOUR ON CHARACTERS CONTAINING "1", IN THE PARTICULAR CELL
            }
         }
      }
   }
}