From eab2b17d640b44cf44dc6f2a2f35af465cb694ee Mon Sep 17 00:00:00 2001
From: Andrew Murray <radarhere@users.noreply.github.com>
Date: Wed, 30 Jun 2021 00:05:25 +1000
Subject: [PATCH 1/2] Do not use background or transparency index for new color

---
 Tests/test_imagepalette.py | 19 +++++++++++++++++++
 src/PIL/ImagePalette.py    | 10 +++++++++-
 2 files changed, 28 insertions(+), 1 deletion(-)

diff --git a/Tests/test_imagepalette.py b/Tests/test_imagepalette.py
index ee2cfdf9b..9c4f8acfb 100644
--- a/Tests/test_imagepalette.py
+++ b/Tests/test_imagepalette.py
@@ -49,6 +49,25 @@ def test_getcolor():
         palette.getcolor("unknown")
 
 
+def test_getcolor_not_special():
+    im = Image.new("P", (1, 1))
+    for index, palette in {
+        # Test when the palette is not full
+        0: ImagePalette.ImagePalette(),
+        # Test when the palette is full
+        255: ImagePalette.ImagePalette("RGB", list(range(256)) * 3),
+    }.items():
+        # 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)
diff --git a/src/PIL/ImagePalette.py b/src/PIL/ImagePalette.py
index 76c4c46d5..b0c722b29 100644
--- a/src/PIL/ImagePalette.py
+++ b/src/PIL/ImagePalette.py
@@ -118,11 +118,19 @@ class ImagePalette:
                 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:

From 8f5762ec5f739c5b99f295179870361588d12f23 Mon Sep 17 00:00:00 2001
From: Andrew Murray <radarhere@users.noreply.github.com>
Date: Wed, 30 Jun 2021 20:32:48 +1000
Subject: [PATCH 2/2] Parametrized test

---
 Tests/test_imagepalette.py | 32 ++++++++++++++++++--------------
 1 file changed, 18 insertions(+), 14 deletions(-)

diff --git a/Tests/test_imagepalette.py b/Tests/test_imagepalette.py
index 9c4f8acfb..ecfbda1d8 100644
--- a/Tests/test_imagepalette.py
+++ b/Tests/test_imagepalette.py
@@ -49,23 +49,27 @@ def test_getcolor():
         palette.getcolor("unknown")
 
 
-def test_getcolor_not_special():
-    im = Image.new("P", (1, 1))
-    for index, palette in {
+@pytest.mark.parametrize(
+    "index, palette",
+    [
         # Test when the palette is not full
-        0: ImagePalette.ImagePalette(),
+        (0, ImagePalette.ImagePalette()),
         # Test when the palette is full
-        255: ImagePalette.ImagePalette("RGB", list(range(256)) * 3),
-    }.items():
-        # Do not use transparency index as a new color
-        im.info["transparency"] = index
-        index1 = palette.getcolor((0, 0, 0), im)
-        assert index1 != index
+        (255, ImagePalette.ImagePalette("RGB", list(range(256)) * 3)),
+    ],
+)
+def test_getcolor_not_special(index, palette):
+    im = Image.new("P", (1, 1))
 
-        # 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)
+    # 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):