Like leaves in autumn, they can change to orangish, reddish, purplish, yellowing..once you harvest, its kind of how you want the plant looking, it has used up all its stored nutrients in lower leaves, the buds won't come out tasting 'chemically'. As for being a huge waste if things go wrong..it really isn't a waste in the longrun, as it's only through mistakes/failures, that one actually ends up learning something
My first grow a year and a half ago, and 2 auto plants got me 1oz total of far below average quality..i still had to buy from my guy as what i grew was weak! Fast forward..my last grow this summer netted me just shy of 12 oz's between 4 auto's (using just small cfl's and an old mars 300), half of it above average, the other half average, but it seems the strain wasn't great from online reviews, so the quality of the blue mammoth, biggest one, was average. It takes time, there are many things to know, most especially when something starts going wrong, and no matter how good you are..the best of the best will still run into issues from time to time, it happens..knowing what to do is key, and you only know because you had to go through it once before
I have spent hundreds and hundreds of hours reading mostly agricultural papers from university studies, the science of how plants grow, how they eat, how they treat each element, how the elements behave in soil. If you care and have the time, i suggest starting with a search on anions and cations in soil, it can sound complicated, but from there you may find your path. Im still learning alot! lol