Comparing database values with values from a GridView
It looks like what you want is ArrayList.Contains.

string rowHostName = row.Cells["HostName"].Value.ToString();
string rowUsers = row.Cells["Users"].Value.ToString();
string rowPSName= row.Cells["PS_NAME"].Value.ToString();

if (arrHostName.Contains(rowHostName) 
    || arrUsers.Contains(rowUsers) 
    || arrPSName.Contains(rowPSName))
{ 
    row.Appearance.BackColor = Color.Yellow;
    row.Appearance.FontData.Bold = Infragistics.Win.DefaultableBoolean.True;
}


And if you only use the collections above to check if they contain some values, you should prefer HashSet over ArrayList.

Of course, you should then change the names of the local variables accordingly, from arrHostName to hostNames and so on. It is always better not to clutter variable names with type information in C#, whose type system is good enough.