Not a bag thing at all. They can turn a rainbow of colors. It indicates stored energies being used up. Exhausting certain nutes will change certain colors. (In your case, I believe phosphorus) they're doing their job, supplying nutes to the buds for the final finale. That also tells you the stored nutes are being used up so you don't taste them in your smoke!
-- I've read Pappapain's alert on my gmail, but haven't clicked onto his post yet. If you haven't been there yet, GO! Awesome pics, awesome grow, & his girls are turning purple. I'm getting ready to click over & have a look myself. (Soldiers co-op grow).