I spent a while yesterday trying to identify my little yellow baetis nymphs, but am still none the wiser. There seems to be a paucity of information about the colour of riverfly nymphs generally. I have some pretty comprehensive field guides that should enable identification down to species level, but these rarely mention colour as an identifier so maybe colours vary within species depending on habitat? I shall persevere.
I forgot to post up a photo sent to me of one of our early season trout (sorry Rob) so here it is just to whet your appetite.

Ian