The flag is red, white, and blue.
Most grammer handbooks note that the comma before the conjunction is optional.
The best way to see the country, unless you are pressed for time, is to travel on foot. Enclose parenthetic expressions between commas. A parenthetic expression is part of the sentence that could be omitted without changing the meaning of the sentence or making the sentence ambiguous. If the expression is a necessary part of the sentence, don't set it off with commas. Dates usually contain parenthetic words or numbers.