with springs i find that if they still have a chrome look to them then the meat will be tough still. what i mean by this is if the fish looks grey but is still nice and shiny then its only starting to get grey and is infact just barely turning. alot of the springs i have been seeing lately are getting grey but still shiny. I hooked a couple the other day that were chrome flashing around in the water but when landed they had a bit of grey to them
remember that fish will colour slightly from the stress of the battle especially when it takes a bit to land which springs tend to take LOL
Yes the squeeze test can work with every species, i find it tougher on springs because their meat is just so thick that it takes a really hard squeeze to check them which i will not do to an alive fish. its best to just use other ways of telling like i stated already.
I mean in all seriousness if you want to take fish then use your own judgement for them. If you land it and dont like the look the let it go and keep fishing for a brighter one. We have all done this, I remember weeding through countless coho just to get bright ones, i must have caught a good 50 of them just for the 2 chromers i took home that day.