Output all the white or black squares of a chessboard
Pyth, 22 21 bytes
-1 byte by @Sp3000
fn%Chz3%sCMT2sM*&lt;G8S8

Under the function %Chz3, dark hashes to 1, light to 0, and both to 2. If we take the parity of the sum of the ords of a chess square (that is, a1 -&gt; [97, 33] -&gt; (97 + 33)%2 = 0, dark squares go to 0, and light to 1. This allows us to filter by inequality.
fn%Chz3%sCMT2sM*&lt;G8S8      implicit: z=input
               *           Cartesian product of
                &lt;G8          first 8 letters in G (alphabet)
                   S8        with [1,...,8] implicitly stringified
             sM*&lt;G8S8      ['a1','a2,...,'a8','b1'...'h8']
f          T               Filter that by gives truthy result to lambda T:
        sCMT                   The sum of the ords of the chars in T,
       %    2                  modulo 2
 n                            does not equal
   Chz                          ord of the first char in z,
  %   3                         modulo 3
                            Implicitly print the list.

Try it here.