OK, so I decided I'm going to re-dye my hair back to it's "Extremely Loud Rastastripes" as my friends call it. It's permanently black in the back, but I did the sides in green(which I mix myself from yellow and blue, because all the pre mixed greens suck tintbrush), yellow, and red last time, and I want to do it again.

One problem: I just looked in my haircolor bag, and noticed that I no longer have any yellow Manic Panic. Crap. I still have about half a jar of blue, and enough red.

The only yellow I have is Color Fiend yellow, which I have noticed is very pale and weak. I have never tried mixing it with manic panic, and I wonder, what are the chances of it either turning out funky and ugly, or making my hair fall out via some weird chemical reaction?

Doubt there'd be any disastrous chemical reactions. Hair only falls out if you keep dying it month after month so you're pretty safe on both accounts and worst case scenario you could always redye it.
<3