Red Spring Onions are Green Onions (aka Spring Onions) that are reddish-coloured (actually more purple.)
The stalk is white at the tip of its base, turning red past that, and then green at the top.
The red colour stays when cooked, and becomes somewhat brighter.
They have a sweet and mild-flavored taste like green onions, but look great in dishes.
Substitutes
Green onions