See the following arrangement, where 0 stands for the other 2 colors and 1 for red;
0 1 0
1 0 1
0 1 0
0 0 0 1 0 1 0 0 0 0 0
1 0 1 0 1 0 1 0 1
0 0 1 0 1 0 0 0 0 0 0
0 1 0
1 0 1
0 1 0
1 0 1
0 1 0
1 0 1
Count the number of 1's, that is the number of red squares