Alternating Colours (Lyra)

Hoping to be able to adjust a theme element within our site. For mobile we would like to alternate the color sequence slightly to make site more readable.

For the main page we would like to switch the second section (posts four through six) and make them a slightly lighter color of blue.

Inform us please of any information that would allow us to achieve this goal. (We are implementing Lyra as a theme.)