I think that Queen Mary knows exactly what she's doing. She does not disappear at all imo but she also does not stand out too much as to overtake attention away from King Frederik or the Swedish royal couple as the hosts (if that makes sense). She stans out against Frederik's dark uniform but she is not taking any attention away from him as the monarch. She also does not clash with what anyone else is wearing either. I think she struck the perfect balance. And yes, she shows that even as Queen she did not need to go out and buy a whole new bespoke wardrobe and can re-wear things from her closet.