Doppler is a knife-only finish. Valve rolls a pattern index; traders group those indices into Phase 1–4 plus the gem finishes Ruby, Sapphire and Black Pearl (and Emerald on Gamma Doppler).
Phase 1 leans black/red, Phase 2 pink-red, Phase 3 green-blue, Phase 4 bright blue. Gems are the sparse indexes that stay almost a single jewel color. A Sapphire Butterfly is not “the same skin” as a P3 Butterfly in the market’s eyes.
Dust2pedia prices on a Doppler listing are usually the cheap-phase floor. Gem and max-blue / max-red copies trade on inspect markets (CSFloat pattern filters). Use the knife’s skin page for the floor, then pay the pattern premium only if the inspect matches the phase you want.
Related skins
See also pattern index and overpay.