Fix reading XPM hex colours that do not have 2 digits per channel

Tests/test_file_xpm.py

@@ -52,6 +52,27 @@ def test_transparency() -> None:
     assert converted.getpixel((2, 0)) == (0, 0, 255, 255)
 
 
+@pytest.mark.parametrize(
+    "colour, expected",
+    (
+        (b"#F00", (240, 0, 0)),
+        (b"#FF8000", (255, 128, 0)),
+        (b"#123456789", (18, 69, 120)),
+        (b"#C71BC30BC71B", (199, 195, 199)),
+        (b"#FFFF00000000", (255, 0, 0)),
+    ),
+)
+def test_hex_colour_lengths(colour: bytes, expected: tuple[int, int, int]) -> None:
+    data = b"""/* XPM */
+static char *test[] = {
+"1 1 1 1",
+"a\tc %s",
+"a"};
+""" % colour
+    with Image.open(BytesIO(data)) as im:
+        assert im.convert("RGB").getpixel((0, 0)) == expected
+
+
 def test_transparency_rgba() -> None:
     # With more than 256 colours and a transparent colour,
     # the image is opened as RGBA
@@ -77,6 +98,26 @@ def test_truncated_header() -> None:
             pass
 
 
+@pytest.mark.parametrize(
+    "colour, expected", ((b"#1234", (0, 18, 52)), (b"#12345678", (52, 86, 120)))
+)
+def test_other_hex_colour_lengths(
+    colour: bytes, expected: tuple[int, int, int]
+) -> None:
+    # Other lengths keep using the lowest 24 bits, without shifting the next colour
+    data = b"""/* XPM */
+static char *test[] = {
+"2 1 2 1",
+"a\tc %s",
+"b\tc #00ff00",
+"ab"};
+""" % colour
+    with Image.open(BytesIO(data)) as im:
+        rgb = im.convert("RGB")
+        assert rgb.getpixel((0, 0)) == expected
+        assert rgb.getpixel((1, 0)) == (0, 255, 0)
+
+
 def test_cannot_read_color() -> None:
     with open(TEST_FILE, "rb") as fp:
         data = fp.read().split(b"#")[0]

src/PIL/XpmImagePlugin.py

@@ -78,6 +78,16 @@ def _open(self) -> None:
                     if rgb == b"None":
                         transparent_key = c
                         palette[c] = b"\0\0\0"
+                    elif rgb.startswith(b"#") and len(rgb) in (4, 7, 10, 13):
+                        # #RGB, #RRGGBB, #RRRGGGBBB or #RRRRGGGGBBBB
+                        # libXpm parses colours with XParseColor,
+                        # which uses the digits as the most significant bits
+                        # of each channel instead of scaling them
+                        digits = (len(rgb) - 1) // 3
+                        palette[c] = bytes(
+                            int(rgb[i : i + digits], 16) << (16 - 4 * digits) >> 8
+                            for i in range(1, len(rgb), digits)
+                        )
                     elif rgb.startswith(b"#"):
                         rgb_int = int(rgb[1:], 16)
                         palette[c] = (