I have made some Hi-Vis yellow P.U. Nylon stuffbags and rucksack liners.
These are the only lairy coloured bits of gear I have, and were made up precicely because I would be so difficult to see, if I had an accident on the Hill.
I HATE seeing bright red/orange/yellow/blue "dots" all over the Hills, as all the "experts" pose around in their £400.00+ Gore-Tex gear!
Back on the subject of signals, there are also Strobes and Flares available.
A Strobe is probably going to be ignored by pretty much everybody, except in the Mountains........where their range is pretty questionable, especially in bad weather.
Flares are also of very limited use (especially "Pen Flares" ) as they have a very short burn time, plus they will either be ignored as "fireworks" by most people, or in the Mountains be completely invisible after you launch them into the overhead clouds!
As for Ground to Air signals......First of all it requires a Military trained pilot to understand them in the first place!
Secondly, it is very unlikely that even an Ex-Military pilot flying in the UK is going to bother to look for Ground to Air signals anyway.
For this reason they are of no practical value at all.
Quite apart from needing to be at least 5 metres long at the minimum (and who is going to make them if you are injured anyway?) and they also require huge amounts of effort to make.....time which could be spent much more effectively.