Adult male Blue-tailed Damselflies (Ischnura elegans) have a head and thorax patterned with blue and black.
Juveniles may be salmon pink (form rufescens), violet (form violacea) or pale green (form infuscans). 
The colour darkens as the damselfly ages, slowly turning to the blue of most mature adults.
Female blue-tailed Damselflies come in a variety of colour forms.
Mature females may be blue like the male (form typica), olive green (form infuscans) or pale brown (form infusca-obseleta).

Knowing that the abdomen of a damselfly consists of 10 segments, the Blue-tailed Damselfly can be distinguished from the similar looking Scarce Blue-tailed Damselfly (Ischnura pumilio) by having an entirely blue 8th segment while the Scarce Blue-tailed Damselfly has an entirely blue 9th segment.

1st photo: Siertuin Gooilust, 's Graveland, The Netherlands, 27 July 2016
2nd photo: Zanderij Crailo, Hilversum, The Netherlands, 24 June 2013
3rd photo: A female. Spandersbos, Hilversum,The Netherlands, 22 July 2016
4th photo: Loosdrecht, The Netherlands, 10 July 2011
5th photo: Boekel area, The Netherlands, 6 July 2018