This might be of interest to anyone who wants to go deeper into the feeding habits of the different age classes. From there on you could deduct what type of fly would be better for grilse, 2SW and 3SW...

Feeding habits of wild and escaped farmed Atlantic salmon, Salmo salar L., in the Northeast Atlantic ? ICES J. Mar. Sci.

This picture struck me as well, proving why yellow and orange/red work so well.

Best regards,