Using that rule of thumb, you would successfully avoid Amanita virosa (destroying angel), but you could still get poisoned by Amanita phalloides (death cap) which has a pale-green cap.

I don't endorse any rules-of-thumb for foraging, but a much better differentiator for beginners is whether the mushroom has pores vs. gills under the cap. There are virtually no deadly varieties that have pores.