Handle the premultiplied La and RGBa modes in ImageColor.getcolor()

Both alpha checks test mode[-1] == "A", uppercase only, so the premultiplied
modes La and RGBa fall through and their alpha is dropped: Image.new("La", s,
"red") comes out fully transparent, and RGBa silently forces alpha to 255.

Premultiply with rounding to match convert(); verified against convert() over
all 256 alpha values.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: chuenchen309

Tests/test_imagecolor.py

@@ -199,6 +199,19 @@ 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", ("red", "#ff000080"))
+def test_color_premultiplied(mode: str, premultiplied_mode: str, color: str) -> None:
+    # The premultiplied modes are spelled with a lowercase "a", so an
+    # uppercase-only check drops their alpha. Compare against convert().
+    expected = (
+        Image.new(mode, (1, 1), color).convert(premultiplied_mode).getpixel((0, 0))
+    )
+
+    assert ImageColor.getcolor(color, premultiplied_mode) == expected
+    assert Image.new(premultiplied_mode, (1, 1), color).getpixel((0, 0)) == expected
+
+
 def test_color_too_long() -> None:
     # Arrange
     color_too_long = "hsl(" + "1" * 40 + "," + "1" * 40 + "%," + "1" * 40 + "%)"

src/PIL/ImageColor.py

@@ -159,12 +159,21 @@ def getcolor(color: str, mode: str) -> int | tuple[int, ...]:
         graylevel = (r * 19595 + g * 38470 + b * 7471 + 0x8000) >> 16
         if mode[-1] == "A":
             return graylevel, alpha
+        if mode[-1] == "a":
+            return _premultiply(graylevel, alpha), alpha
         return graylevel
     elif mode[-1] == "A":
         return rgb + (alpha,)
+    elif mode[-1] == "a":
+        return tuple(_premultiply(band, alpha) for band in rgb) + (alpha,)
     return rgb
 
 
+def _premultiply(band: int, alpha: int) -> int:
+    # Rounded, to match convert()'s premultiplication.
+    return (band * alpha + 127) // 255
+
+
 colormap: dict[str, str | tuple[int, int, int]] = {
     # X11 colour table from https://drafts.csswg.org/css-color-4/, with
     # gray/grey spelling issues fixed.  This is a superset of HTML 4.0