Working with an artist who wishes to use Photoshop, RGB mode
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Can't spell Photoshop without photos
Photoshop assumes you're working on photos. Edges in photos are soft due to imperfect focus, interpolation in the camera to compensate for Bayer-dithered image sensors, and the nonzero size of pixels in the image sensor. The JPEG lossy compression used on photos introduces additional artifacts along edges, where colors are close but not exact to the intended color. Fuzzy fill algorithms fill the color under the action point and additional colors close to it, possibly proportionally mixing in pixels whose color is far enough from the original color. It sounds like Photoshop is using at least such a fuzzy fill. GIMP can use one or not depending on the Tool Options.