We have lots of dragonflies around the reservoir near where I live (60km north of Madrid). Recently I photographed what I thought was a red veined darter but on inspection it was clearly different from previous specimens.
Most of the red veined darters I've photographed are overall red and look like the image in my field guide (Chinery)...
This spring's specimen had a yellow flash across the thorax and a colourful collar behind the head...
Is this also a red veined darter (as I suspect)? Is this colouration special to the beginning of the season? Or, to put it most generally, if these are both the same species why the noticeable difference?
I don't see any additional colouration, both individuals are identical. Anyway, sometimes there are not so brigthly coloured males, with only a shade of red in abdomen, these are inmature males.
Both specimens have the colours that you mentioned. All mature males of Sympetrum fonscolombei have a yellow splash on the sides of thorax, and you can see it in both images (of course more clearly in the one taken in the right angle and not shaded). And all the individuals have the rear of the head like this - but obviously it can not seen if you don't photograph from an angle where the rear of the head can be seen!