Minecraft CanBreak on specifically colored banners

I'm going to have to disappoint you, as it is not possible to only allow players to destroy a banner with a certain color. Either a player can destroy all banners, or non at all.

An alternative would be to place an entity with a colour of your preference and add it to the team who is not allowed to break it. The opposite team can kill that entity, in order to "break the flag".

EDIT

So I've been doing some testing. I've replaced "minecraft:banner" with {id:"minecraft:banner"}, but that didn't work. I've tried to omit the minecraft: part, but that didn't work either. I've tried with and without quotes, but no result. Lastly, I tried to make it work on dirt, like this: CanDestroy:["minecraft:dirt"] and that did work. This means that your syntax is not the problem. This appears to be a bug in the game.