Handle premultiplied alpha modes in ImageColor.getcolor() (#9797)

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>
Co-authored-by: Andrew Murray <radarhere@users.noreply.github.com>

Author: 3 people

Tests/test_image_transform.py

@@ -94,7 +94,7 @@ def test_quad(self) -> None:
         (
             ("RGB", (255, 0, 0)),
             ("RGBA", (255, 0, 0, 255)),
-            ("LA", (76, 0)),
+            ("LA", (76, 255)),
         ),
     )
     def test_fill(self, mode: str, expected_pixel: tuple[int, ...]) -> None:

Tests/test_imagecolor.py

@@ -199,6 +199,18 @@ def test_color_hsv() -> None:
     assert (170, 255, 255) == ImageColor.getcolor("hsv(240, 100%, 100%)", "HSV")
 
 
+@pytest.mark.parametrize("mode, premultiplied_mode", (("LA", "La"), ("RGBA", "RGBa")))
+@pytest.mark.parametrize("color", ("#f00", "#ff000080", "#ff000050"))
+def test_color_premultiplied_alpha(
+    mode: str, premultiplied_mode: str, color: str
+) -> None:
+    im = Image.new(mode, (1, 1), color)
+    im_premultiplied = im.convert(premultiplied_mode)
+    expected = im_premultiplied.getpixel((0, 0))
+
+    assert ImageColor.getcolor(color, premultiplied_mode) == expected
+
+
 def test_color_too_long() -> None:
     # Arrange
     color_too_long = "hsl(" + "1" * 40 + "," + "1" * 40 + "%," + "1" * 40 + "%)"

src/PIL/ImageColor.py

@@ -152,17 +152,19 @@ def getcolor(color: str, mode: str) -> int | tuple[int, ...]:
         r, g, b = rgb
         h, s, v = rgb_to_hsv(r / 255, g / 255, b / 255)
         return int(h * 255), int(s * 255), int(v * 255)
-    elif Image.getmodebase(mode) == "L":
+    if Image.getmodebase(mode) == "L":
         r, g, b = rgb
         # ITU-R Recommendation 601-2 for nonlinear RGB
         # scaled to 24 bits to match the convert's implementation.
         graylevel = (r * 19595 + g * 38470 + b * 7471 + 0x8000) >> 16
-        if mode[-1] == "A":
-            return graylevel, alpha
-        return graylevel
-    elif mode[-1] == "A":
-        return rgb + (alpha,)
-    return rgb
+        value: tuple[int, ...] = (graylevel,)
+    else:
+        value = rgb
+    if mode[-1] == "a":
+        value = tuple(round(band * alpha / 255) for band in value)
+    if mode[-1] in ("A", "a"):
+        return value + (alpha,)
+    return value[0] if len(value) == 1 else value
 
 
 colormap: dict[str, str | tuple[int, int, int]] = {