I wouldn't even write such a class name. It smells like bad styling and layouting already. A CSS class should convey some semantic meaning. I would name it after the thing that should be red or green, not "red"/"green". That doesn't tell me anything. Maybe its name could be something like "danger" or "active" or something. Also the 500 looks very sus. Responsive design is best when it avoids such hardcoded numbers and depends on its content and dynamic viewport width calculations, and perhaps a few minimum sizes, if necessary to decide when things float, wrap, shrink, grow, etc.

It's a red flag (ha).