I usually make a 2 element array, and store one color in each element. Then I have a flip-flop counter, that starts at zero or one, and at the end of the loop, is set to one minus itself (thus flip-flopping between 0 and 1), and then in the loop just refer to the counter's element of the array. It goes something like this:

@colors = (black, white);
$flipflop=0;
while ($n < $nmax) {
  print("<tr color=\"$colors[$flipflop]\"><td>content</td></tr>");
  $flipflop = 1 - $flipflop;
}
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