title: Changing the values of every second row (pandas data frame)
body: <p>I would like to change the values of the rows of the first column to 'green' for every second value and 'red' for the remaining values (see below, where I started doing it with the replace method, but it is not very efficient.</p>
  
  <pre><code>    color   IntDen  Density Condition
  0   green   936645  Low Ctrl
  1   red 1571204 Low Ctrl
  2   green   1154038 Low Ctrl
  3   red 1423050 Low Ctrl
  4   5   691283  Low Aß42
  5   6   764449  Low Aß42
  6   7   638492  Low Aß42
  7   8   887382  Low Aß42
  8   9   788693  Low H20
  9   10  1401201 Low H20
  10  11  936645  Low H20
  11  12  1571204 Low H20
  12  13  1122389 Low LPS
  13  14  2262593 Low LPS
  14  15  866009  Low LPS
  15  16  1398048 Low LPS
  16  17  960636  High    Ctrl
  17  18  1939803 High    Ctrl
  18  19  1994001 High    Ctrl
  19  20  3399789 High    Ctrl
  20  21  810265  High    Aß42
  21  22  1113727 High    Aß42
  22  23  1095145 High    Aß42
  23  24  1402325 High    Aß42
  24  25  1424273 High    H20
  25  26  2771698 High    H20
  26  27  1807418 High    H20
  27  28  1265651 High    H20
  28  29  2046300 High    LPS
  29  30  1225662 High    LPS
  30  31  1755320 High    LPS
  31  32  1425662 High    LPS
  </code></pre>