Do not use background or transparency index for new color

Tests/test_imagepalette.py

@@ -49,6 +49,29 @@ def test_getcolor():
         palette.getcolor("unknown")
 
 
+@pytest.mark.parametrize(
+    "index, palette",
+    [
+        # Test when the palette is not full
+        (0, ImagePalette.ImagePalette()),
+        # Test when the palette is full
+        (255, ImagePalette.ImagePalette("RGB", list(range(256)) * 3)),
+    ],
+)
+def test_getcolor_not_special(index, palette):
+    im = Image.new("P", (1, 1))
+
+    # Do not use transparency index as a new color
+    im.info["transparency"] = index
+    index1 = palette.getcolor((0, 0, 0), im)
+    assert index1 != index
+
+    # Do not use background index as a new color
+    im.info["background"] = index1
+    index2 = palette.getcolor((0, 0, 1), im)
+    assert index2 not in (index, index1)
+
+
 def test_file(tmp_path):
 
     palette = ImagePalette.ImagePalette("RGB", list(range(256)) * 3)

src/PIL/ImagePalette.py

@@ -118,11 +118,19 @@ def getcolor(self, color, image=None):
                 if not isinstance(self.palette, bytearray):
                     self._palette = bytearray(self.palette)
                 index = len(self.palette) // 3
+                special_colors = ()
+                if image:
+                    special_colors = (
+                        image.info.get("background"),
+                        image.info.get("transparency"),
+                    )
+                while index in special_colors:
+                    index += 1
                 if index >= 256:
                     if image:
                         # Search for an unused index
                         for i, count in reversed(list(enumerate(image.histogram()))):
-                            if count == 0:
+                            if count == 0 and i not in special_colors:
                                 index = i
                                 break
                     if index >= 256: