[Vena]

It just adds a fun dynamic to make Fushimi take up Blue, while Yata takes up Red (assuming they even go that way). I can just as easily (and more easily support) a grown up Anna being the Red king.

Spoiler for Another note on Shiro:

I'm going to assume that what "Only kings can kill kings." refers to is removing Colorless. Shiro as Silver never displays any offensive abilities, he and his abilities (and his personality) are extremely passive. All he can do is restrain colorless whom he allowed to cause the whole mess while Red kills him.

On top of all of that, Shiro flatout says: 「ぼく わ ふし の おう」
Lit: "I am the immortal king."

So long as the Dresden plate exists or he doesn't renounce his kingship, I don't think Weismann's spirit can ever die. The body is probably gone but Weismann is still there... in the umbrella.

Edit:
@Kuro: The umbrella survived. Maybe he's in the umbrella?

We do have a convenient Weismann body floating around, and he's been stuck at 20-something for the last fifty odd years. And he sort of looks like Shiro too...