With Impossible film you should set the Lighten / Darken wheel to about two thirds to the dark, that will give you much better pictures. The nuisance about the SX-70 is that this wheel returns to the mid position every time you close it. The other thing you have to observe is to shield the picture from light for the first seconds after it ejects. The latest films have improved on this issue, but it could still be a problem in strong light. Impossible has a special frog tongue for most Polaroid models that will cover the whole picture, I can recommend it. Otherwise you can use the darkslide from the film pack to shield the picture.
Using a Polaroid camera that do not have a Lighten / Darken adjustment with Impossible film is simply not recommended.
Good to know. I did turn the dial as far as it would go towards dark and I used my hand to shield the print as it came out of the camera. As soon as I could, I pulled it out of the camera and stuck it into a drawer. The last four shots that I took this way all ended up blurry, like motion blur, and they all still have that yellowish-cast.
I haven't scanned them all in yet, but I was going to start a thread to see what, if anything, could be sorted out to determine how much of the problem was a) the film, b) the camera, c) my error.
I wish it weren't so expensive