How do they make this effect, where grayscale becomes RGB?

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The pink and green on her necklace and the edges of her hair aren't gray like the rest of the picture, but they blend well because they have similar values. Zoom in, and you'll see they're actually pink and green.

It's called chromatic aberration. In real photos, this is usually due to prismatic effects of the material that the camera lens is made out of.

You can make a cheap simulation of the effect in an image editor by separating the RGB channels of the layer and moving them slightly.

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