The tint of firefly bushes is not affected by biome temperature at all, unlike all similar plants which change their color slightly to accommodate to the climate according to the biome they're in.

steps to reproduce:

1. Locate any cold biome (such as the Snowy Plains or the Frozen Peaks), and place a firefly bush there.
2. Locate any hot biome (such as a Desert or a Savannah), and place a firefly bush there.
3.  Observe whether there is any difference in the tint of the plant.