"Red against blue."
Translation:Κόκκινο εναντίον μπλε.
Its usage is different. It cannot be used in syntax "A vs B".
However, you can use it in sentences like "The war/battle/etc. against B": "Ο πόλεμος κατά του μπλε" - "The war against blue". Note "μπλε" in genitive case and with definite article.
Same meaning, different preposition, different syntax: "Ο πόλεμος ενάντια στο μπλε".