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.
Note that there's at least one, and that a number of them will still make you sick.
Put a few common pored edibles (Laetiporus sp., Cantharellus sp., Grifola frondosa, etc.) in your notes for sure though, with basic descriptions, as even their few remote lookalikes (at least pored ones -- some people think Omphalotus sp. look like Cantharellus for some reason) are just going to give you a moderate stomach upset at worst, and may be edible themselves.
