Funnily enough, "watermelon" is either a slur or a term of endearment for Green Party members and leadership in the U.S., depending on who's using it or who it's referring to.
A lot of Amerikan comrades likely have some amount of disdain for Jill Stein and even Ajamu Baraka, for some justified reasons and some reasons that stem, I feel, purely from anti-socialist propaganda manufactured to sabotage them from both the left and the right. They're watermelons, green on the outside and red on the inside. I had the wonderful pleasure of meeting comrade Ajamu in person in 2019 or so. The man is undoubtedly a communist and an anti-imperialist. I met Dr. Stein, too, and she's quite wonderful albeit with perhaps a more optimistic and maybe naive belief in "pan-leftist" organizing as it were.
I don't think the greens are the vanguard. I don't think the Green Party as it exists can or ever will function as one. But I implore comrades who dislike them for surface level reasons to examine if those reasons actually make sense or are justified.
(I didn't mean to hijack your post in support of the Amerikan green party lol. From my understanding, the German greens are filled to the brim with fucking pedophiles. They're brown and rotten watermelons all the way through.)