Your code will affect all the cells in every fourth row.
What you want is for the header row to alternate color every other column, and the data rows to alternate the 4th, 5th and 6th of every 6 columns.
thead tr th:nth-child(2n) {
background-color: #ccf;
}
tbody tr th:nth-child(6n + 4),
tbody tr th:nth-child(6n + 5),
tbody tr th:nth-child(6n + 6),
tbody tr td:nth-child(6n + 4),
tbody tr td:nth-child(6n + 5),
tbody tr td:nth-child(6n + 6) {
background-color: #ccf;
}