Hyale and the Berger's Clouded Yellow (Colias alfacariensis) are almost impossible to tell apart by external characteristics. It is probably true that any individual that appears bright lemon yellow is almost certainly (male) alfacariensis, but the reverse is not necessarily true, and this pointer may be subject to considerable regional and altitude variation.

The most reliable (if any are) external characteristic is that alfacariensis has a slightly more rounded forewing outer margin and hyale is straighter with a more pointed wing tip (apex). If either the margin or apex are rounded, it is alfacariensis, but I'm not sure the converse is always true. The degree of curvature seems to me to be very variable for alfacariensis, some being extremely rounded and others either quite straight-edged and/or quite pointed at the apex. Photographs can be misleading if not exactly at 90 degrees to the plane of the insect.

The upperside uph discal spots are a different colour, but this doesn’t help (unless you are prepared to catch and examine specimens, which I am not) as they do not settle with open wings, although occasionally a quick photo of courtship might catch this.

It is easy to separate them in Var: hyale does not occur in Var, according to Lafranchis, and I have not seen any concrete evidence to disbelieve this. I have, however, been looking closely at the forewing margins and apices of specimens in northern Var in 2008 and they are decidedly less rounded than those seen in the south of the département. Northern Var is a comparative wilderness compared to the relatively highly populated south of Var (the A8 autoroute makes a good dividing line), and I suspect somewhat under-recorded.

observations

I believe this to be a male hyale largely on the grounds of the forewing shape and the rather heavily dusted unh. It was seen in the Hautes-Alpes where hyale is known to occur, albeit a few km from the nearest recorded sighting. However, alfacariensis is more widespread in the Hautes-Alpes and has an upper altitude limit of 2400m according to Lafranchis, whereas hyale is limited to 2000m although these ranges should not be interpreted rigidly.

I believe this is a male hyale as the forewing outer margin is quite straight and the apex is maybe just about pointed enough for hyale. The ground colour is a pale lemon yellow indicating hyale. It was seen in northern Italy, where hyale does occur.

I believe this could be a male hyale because of the colouring and the evident straight margin and pointed wingtip. However, the butterfly was in motion, so this may not be conclusive.