What would a red filler do to red hair? what happens if i mix it with the hair dye?

It may turn it more red, depending if your hair is blondish-red or brownish-red. Depending on the hair dye, it may just stay the same.

I suggest you just leave your the hair it is, or maybe add a few highlights here and there =)

What would a red filler do to red hair? what happens if i mix it with the hair dye?

Any number of things. It depends on the hair color (dye) and the shade of the filler, and the base color in each as well as what color your hair is now. Don't do it yourself !

What would a red filler do to red hair? what happens if i mix it with the hair dye?

i wouldnt ever dye ur hair urself, most of the time u mess up. or the color doesnt turn out right