This is not what she says. She says:
It's like when The Wizard of Oz turns to colour.
The 1939 Wizard of Oz starts in black and white and turns to color when Dororthy reaches Oz. Beverly is basically saying that she's now in Oz, an amazing and colorful wonderland as compared to the drab world outside.
Not all of the Wizard of Oz is in color, only the parts that take place in Oz.
Everything that happens while Dorothy is still in Kansas is shot in black and white. This was of course done to underline how colorful and magical the land of Oz is compared the real-world Kansas.
All of the Oz sequences were filmed in three-strip Technicolor.
The opening ...