The Ruddy Darter (Sympetrum sanguinem) is quite similar to the Common Darter (Sympetrum striolatum) and the Vagrant Darter (Sympetrum vulgatum).
However, the Ruddy Darter can be easily recognized by its completely black legs (clealry visible on the photos).
The other two Darters have yellow or brown stripes along the legs.
All three dragonflies have a black spot on top of their snouts, like a sort of mustache.
In the Vagrant Darter and Ruddy Darter, the black runs down the eyes' edges, turning the mustache into a drooping mustache.
The Common Darter is missing this and so it only has the black mustache on top.

Schlammwiss Nature Reserve, Luxembourg, 13 August 2024