You should:
Loop through the your gridview rows first.
Locate your cell index in your gridview's row.
Find the cell value.
Compare cell value with 50.
If bigger change the color to desired.
Sample code;
for (int i = 0; i < GridView.Rows.Count; i++)
{
try
{
if (Convert.ToInt32(GridView.Rows[i].Cells[CellIndex].Text) > 50)
{
GridView.Rows[i].Cells[CellIndex].BackColor = Color.Green;
}
else
{
GridView.Rows[i].Cells[CellIndex].BackColor = Color.Red;
}
}
catch (Exception)
{
}
}