When I boot, I get a blue screen. When I boot in safe mode, I can't log in.

I have a late 2009 iMac. It has two startup disks, Yosemite, and Snow Leopard. Of course, Snow Leopard had Rosetta which I use to run really old programs. However, when I choose it as a startup disk from system preferences in my Yosemite disk, it takes a really long time to boot, and just ends up in a frozen blue screen. the only way to get out of it and go back to Yosemite is resetting my NVRAM. When I boot Snow Leopard in safe mode, i can actually get to the log in screen, but when I type in my password, the screen goes blue for 3 seconds and then asks for the password again, even though its right.