Quick touch-up Question

I've got a real quick question, here.

I've been dying my hair with Ion products from Sally's.
I've been between the light ash brown and moved up to
medium ash brown, just recently.
I've been very good about only touching up my roots
(as opposed to dying the whole hair all over, like I used
to do). But now, I'm due for another touch-up and it's gonna
be a while before I can get near a Sally's again.

So, since I'm right near a Wal-Mart, I was wondering if it
would be alright for me to just go buy some hair color
from there to touch up my roots. Would that be a good idea
or no?

Thanks