If GIMP changes the color of a cutout but refuses to extend it into the transparent area, inspect Lock Alpha on that image layer. Transparency may already exist and be protected. Adding another alpha channel or increasing brush opacity will not unlock it. First establish whether your intended change is recoloring existing material or creating new visible pixels.
Read the kind of lock
GIMP distinguishes locks for pixels, position and size, visibility, and alpha. An alpha lock protects transparency while permitting color changes where pixels already exist. A pixel lock blocks painting more broadly. The two states can look similar when you judge them from only one failed stroke, so inspect the selected layer rather than guessing from the canvas.
For a product cutout, alpha protection can be useful when changing the color of an existing graphic without painting outside its silhouette. It becomes an obstacle when you deliberately need to add a small missing opaque portion. Unlocking it does not establish what that missing portion should contain. Use the original photograph to decide whether an edge actually needs reconstruction.
A two-stroke diagnostic on a copy
This is a reader test plan, not a completed experiment. Duplicate the cutout document and select its image thumbnail. Place a temporary contrasting background below it so transparent areas are easy to distinguish from white pixels. Clear any unintended selection and record the layer's lock settings before making changes.
Choose a conspicuous brush color. Draw one short stroke that begins inside the cutout and crosses into empty space. Undo it. Change only the alpha lock, then repeat the stroke along the same route with the same brush size and opacity. Compare the interior recoloring and the new area outside the original silhouette separately. That distinction identifies whether the setting governs transparency rather than the brush color.
Decide whether to unlock or preserve
For recoloring a shirt-shaped graphic, keeping the alpha lock can protect a clean boundary. For extending a photographed sleeve, first check the untouched source; painting an arbitrary extension can change the garment's construction. If the apparent missing area is controlled by a layer mask, review that mask instead of modifying otherwise correct source pixels.
If both test strokes fail everywhere, investigate the active layer, pixel lock, layer boundary, and selection. Alpha lock alone does not explain every painting failure. Keep the test on a disposable copy so diagnosis does not become a series of irreversible repairs.
Before using the image in an Imagild photo workflow, export a separate reference and reopen it over two contrasting backgrounds. Accept it when the intended silhouette is intact, no test mark remains, and any added material agrees with the original. These are external GIMP controls; the guide does not imply that Imagild has an identical lock panel.