Does this thesis suggest that they can’t see red at all? Or at least not as a colour other than grey and uninteresting to eat.
As usual, there are conflicting or confusing data on t’interweb thing. This one seems to suggest they can see at least some reds: https://www.boredpanda.com/human-vs-bird-vision/?utm_source=google&utm_medium=organic&utm_campaign=organic
However, it may be a difference as seen in the UV, which I believe pigeons can see (many birds can).
What colour a bird sees may well depend on what type of bird it is, but the normal situation in birds is to have 4 types of cone cells/colour receptors (red, green,blue and UV).
Whatever the cause it is protecting the red lettuces which actually taste just as good or even better than the green ones. Perhaps if general cabbages could be produced that were red too (not just red cabbages which are a bit different) then there would be no need for the miles of netting at the allotments nor the millions of bird scarers in farmers fields