When allocating a new color, repurpose an unused index if necessary

src/PIL/GifImagePlugin.py

@@ -760,7 +760,7 @@ def _get_background(im, infoBackground):
             # WebPImagePlugin stores an RGBA value in info["background"]
             # So it must be converted to the same format as GifImagePlugin's
             # info["background"] - a global color table index
-            background = im.palette.getcolor(background)
+            background = im.palette.getcolor(background, im)
     return background
 
 

src/PIL/Image.py

@@ -978,7 +978,7 @@ class Image:
                         trns_im.putpalette(self.palette)
                         if isinstance(t, tuple):
                             try:
-                                t = trns_im.palette.getcolor(t)
+                                t = trns_im.palette.getcolor(t, self)
                             except Exception as e:
                                 raise ValueError(
                                     "Couldn't allocate a palette color for transparency"
@@ -1016,7 +1016,7 @@ class Image:
                 del new.info["transparency"]
             if trns is not None:
                 try:
-                    new.info["transparency"] = new.palette.getcolor(trns)
+                    new.info["transparency"] = new.palette.getcolor(trns, new)
                 except Exception:
                     # if we can't make a transparent color, don't leave the old
                     # transparency hanging around to mess us up.
@@ -1049,7 +1049,7 @@ class Image:
         if trns is not None:
             if new_im.mode == "P":
                 try:
-                    new_im.info["transparency"] = new_im.palette.getcolor(trns)
+                    new_im.info["transparency"] = new_im.palette.getcolor(trns, new_im)
                 except Exception:
                     del new_im.info["transparency"]
                     warnings.warn("Couldn't allocate palette entry for transparency")
@@ -1764,7 +1764,7 @@ class Image:
             and len(value) in [3, 4]
         ):
             # RGB or RGBA value for a P image
-            value = self.palette.getcolor(value)
+            value = self.palette.getcolor(value, self)
         return self.im.putpixel(xy, value)
 
     def remap_palette(self, dest_map, source_palette=None):

src/PIL/ImageDraw.py

@@ -70,6 +70,7 @@ class ImageDraw:
             self.palette = im.palette
         else:
             self.palette = None
+        self._image = im
         self.im = im.im
         self.draw = Image.core.draw(self.im, blend)
         self.mode = mode
@@ -108,13 +109,13 @@ class ImageDraw:
                 if isinstance(ink, str):
                     ink = ImageColor.getcolor(ink, self.mode)
                 if self.palette and not isinstance(ink, numbers.Number):
-                    ink = self.palette.getcolor(ink)
+                    ink = self.palette.getcolor(ink, self._image)
                 ink = self.draw.draw_ink(ink)
             if fill is not None:
                 if isinstance(fill, str):
                     fill = ImageColor.getcolor(fill, self.mode)
                 if self.palette and not isinstance(fill, numbers.Number):
-                    fill = self.palette.getcolor(fill)
+                    fill = self.palette.getcolor(fill, self._image)
                 fill = self.draw.draw_ink(fill)
         return ink, fill
 

src/PIL/ImagePalette.py

@@ -97,7 +97,7 @@ class ImagePalette:
     # Declare tostring as an alias for tobytes
     tostring = tobytes
 
-    def getcolor(self, color):
+    def getcolor(self, color, image=None):
         """Given an rgb tuple, allocate palette entry.
 
         .. warning:: This method is experimental.
@@ -119,7 +119,14 @@ class ImagePalette:
                     self._palette = bytearray(self.palette)
                 index = len(self.palette) // 3
                 if index >= 256:
-                    raise ValueError("cannot allocate more than 256 colors") from e
+                    if image:
+                        # Search for an unused index
+                        for i, count in reversed(list(enumerate(image.histogram()))):
+                            if count == 0:
+                                index = i
+                                break
+                    if index >= 256:
+                        raise ValueError("cannot allocate more than 256 colors") from e
                 self.colors[color] = index
                 if index * 3 < len(self.palette):
                     self._palette[index * 3] = color[0]

src/PIL/PyAccess.py

@@ -54,6 +54,7 @@ class PyAccess:
         self.image32 = ffi.cast("int **", vals["image32"])
         self.image = ffi.cast("unsigned char **", vals["image"])
         self.xsize, self.ysize = img.im.size
+        self._img = img
 
         # Keep pointer to im object to prevent dereferencing.
         self._im = img.im
@@ -93,7 +94,7 @@ class PyAccess:
             and len(color) in [3, 4]
         ):
             # RGB or RGBA value for a P image
-            color = self._palette.getcolor(color)
+            color = self._palette.getcolor(color, self._img)
 
         return self.set_pixel(x, y, color)