Since you are using bootstrap and you want alternating row colors for every screen sizes you need to write separate style rules for all the screen sizes.
/* For small screen */
.row :nth-child(even){
background-color: #dcdcdc;
}
.row :nth-child(odd){
background-color: #aaaaaa;
}
/* For medium screen */
@media (min-width: 768px) {
.row :nth-child(4n), .row :nth-child(4n-1) {
background: #dcdcdc;
}
.row :nth-child(4n-2), .row :nth-child(4n-3) {
background: #aaaaaa;
}
}
/* For large screen */
@media (min-width: 992px) {
.row :nth-child(6n), .row :nth-child(6n-1), .row :nth-child(6n-2) {
background: #dcdcdc;
}
.row :nth-child(6n-3), .row :nth-child(6n-4), .row :nth-child(6n-5) {
background: #aaaaaa;
}
}
Working FIDDLE
I have also included the bootstrap CSS here.