When the white pads first started showing up, players would say that they make it tougher to spot an opening due to less contrast with the ice and boards. Now that players are used to them I'm sure there is still an effect but probably less than it used to be. It would be hard to quantify with all the moving pieces involved.
Yeah, now that it's become de rigueur in the NHL to wear white pads, there's probably been some conditioning on the part of the shooters to expect them.
AFAIK, the phenomenon is two-fold:
1)
avoiding certain colours: red, yellow, orange. Bright colours that force the eye to notice them and make it easy for shooters to avoid them.
2) not necessarily white in itself, but using white in the design to create an illusion of space. Roy's V2s are a good example of this. It's not white pads on their own that necessarily make a huge difference, but it's using the white creatively to make false holes appear in a shooter's split-second view of the net.
In case anyone forgets Roy's pads, they looked like this: