Just to chime in here, some people over the last year have talked about darkening juice and I've always suspected it's just heat. This has been confirmed by me in the last few weeks. Most of my liquids are completely clear, but after they are in my car all day (90+ degrees lately) they turn darker.. I had one bottle of green apple (completely clear) in my car for a week or so.. it's brown now.
so my guess is that some cartos are designed in such a way that the heat is funneled around the juice, like maybe the conical shape makes the heat concentrated or something, so one carto might turn a juice colors, but still be based on heat.
Just an uneducated guess...
That totally makes sense & I knew heat was a factor but my only exp was with the short wick versions & it seemed to me the taste declines considerably once it really gets to changing color
& even if I clean it, they never have that fresh new taste... Im talking about the stardust here...
However Im on my 3rd day with a Viva & it still taste really good, my juice has slightly gotten a tad darker but no where near the molasses it turns into in a stardust.