Try this
private void dataGridView1_CellFormatting(object sender, DataGridViewCellFormattingEventArgs e)
{
if (this.dataGridView1.Columns[e.ColumnIndex].Name == "yourColumnName")
{
if (e.Value != null)
{
int Value = e.Value;
if (Value<0))
{
e.CellStyle.BackColor = Color.Pink;
}
}
}
}
Hope this helps