Apterae are spindle-shaped and 2.0 - 4.0 mm long. They are usually green, with dark femoral apices ("knees") and long siphunculi that are dark only at the tips.  The sixth segment of each antenna has a terminal process that is 4.2 to 5.2 times the length of the base.

Alates are similar but also have brownish markings on the head and thorax.

Macrosiphum hellebori is similar but is unlikely to be found on Euphorbia.  The sixth segment of its antenna has a terminal process that is 6.4 to 7.9 times the length of the base.

Macrosiphum euphorbiae -- confusingly -- is not particularly associated with Euphorbia but it is widely polyphagous.  It lacks the dark "knees".

Macrosiphum rosae has all-black siphunculi.

Macrosiphum euphorbiellum lives year-round among the leaves of spurge, including the garden varieties Euphorbia amygdaloides and Euphorbia characias.

Rare or under-recorded

Rare or under-recorded

https://influentialpoints.com/Gallery/Macrosiphum_amygdaloides_Euphorbia_aphid.htm