"Traditional" here equals formal. Formal french gardens are a whole things in and of themselves. (Think Versailles.) If your character isn't worried about the rules, then she can grow whatever colors she likes. Yellow and orange french marigolds are a thing for example.

You can probably find books on both formal French gardens and country style French gardens at your local library. If you know what specific town she's in, you can very likely find a list of recommended plants. (In the US this is agriculture extension services attached to a university. I don't know what they are in France.)