After a recent update, I've noticed that the "evolve" button sometimes shows a miniature image of the next evolution of the Pokémon instead of just a silhouette. I'd assumed this was based on whether I'd caught the next evolution of the Pokémon. However, that appears to be wrong, as several Pokémon that I already have still show up as only a silhouette on the "evolve" button.
When/why do some Pokémon show up in full color vs. a silhouette on the "evolve" button?
For example, in the screenshots below, I've caught/evolved multiple Ivysaur, but see the silhouette, while I've only caught one Bayleef, but it shows up in color.