Your slide and frame are two different metals, hence the coloring is different to the finish.

Slide is steel, frame is a lightweight alloy. The frame is actually anodized, which is a little different process than the bluing of the steel slide.

From Merriam Webster:

Anodize: to subject (a metal) to electrolytic action as the anode of a cell in order to coat with a protective or decorative film

From Wikipedia:

Bluing: Bluing is a passivation process in which steel is partially protected against rust, and is named after the blue-black appearance of the resulting protective finish. True gun bluing is an electrochemical conversion coating resulting from an oxidizing chemical reaction with iron on the surface selectively forming magnetite (Fe3O4), the black oxide of iron. Black oxide provides minimal protection against corrosion, unless also treated with a water-displacing oil to reduce wetting and galvanic action. A distinction can be made between traditional bluing and some other more modern black oxide coatings, although bluing is a subset of black oxide coatings.

Nice pistol.