The Sooty Copper (Lycaena tityrus) looks similar to the Large Copper (Lycaena dispar) and the Small Copper (Lycaena phlaeas).
However, the underside of the Large Copper's hind wing is light blue and features an orange band.

​The male Sooty Copper (1st, 2nd & 3rd photo) is distinguished from the Large and Small Copper by having both brown forewings and hindwings.
The female Sooty Copper (4th & 5th photo) is best distinguished by the forewing and hindwing having a sharp orange edge with clear black spots.
In the female Small Copper this orange edge is not nicely filled with sharp black spots and sometimes even very unclear.

1st, 2nd & 3rd photo: A male. Eifel National Park, Dreiborn Plateau, North-Rhine Westphalia, Germany, 9 August 2022
4th & 5th photo: A female. Monschau (near the river Schlüsselbach), North-Rhine Westphalia, Germany, 7 August 2022