I assume (dangerous, I know) that you are riding western by the pads. In that case I have to ask how much does the saddle weigh? How does the saddle fit the horse without the blanket, as that tells you where the pressure points are for your particular saddle, which in turn tells you where your blanket needs to have the most support?
Personally, I find one pad and one saddle that works for the horse then stick with it. That way, you don't have to worry about whether changing out blankets (and therefore support) is effecting the back or not.
IMO, if you have to buy a pad to put under your blanket to get the right fit, why not just spend the money on one really good one? If it is just a color issue, just find one really good pad that fits your horse and saddle (I personally suggest a Classic Equine pad) and get the $5 wool covers, or navajo blankets I guess they are called.
One is closer to God's heart on a horse than anywhere else on earth!