Correct allocating new color to RGBA palette

Tests/test_imagepalette.py

@@ -49,6 +49,12 @@ def test_getcolor() -> None:
         palette.getcolor("unknown")  # type: ignore[arg-type]
 
 
+def test_getcolor_rgba() -> None:
+    palette = ImagePalette.ImagePalette("RGBA", (1, 2, 3, 4))
+    palette.getcolor((5, 6, 7, 8))
+    assert palette.palette == b"\x01\x02\x03\x04\x05\x06\x07\x08"
+
+
 def test_getcolor_rgba_color_rgb_palette() -> None:
     palette = ImagePalette.ImagePalette("RGB")
 

src/PIL/ImagePalette.py

@@ -118,7 +118,7 @@ def _new_color_index(
     ) -> int:
         if not isinstance(self.palette, bytearray):
             self._palette = bytearray(self.palette)
-        index = len(self.palette) // 3
+        index = len(self.palette) // len(self.mode)
         special_colors: tuple[int | tuple[int, ...] | None, ...] = ()
         if image:
             special_colors = (
@@ -168,11 +168,12 @@ def getcolor(
                 index = self._new_color_index(image, e)
                 assert isinstance(self._palette, bytearray)
                 self.colors[color] = index
-                if index * 3 < len(self.palette):
+                mode_len = len(self.mode)
+                if index * mode_len < len(self.palette):
                     self._palette = (
-                        self._palette[: index * 3]
+                        self._palette[: index * mode_len]
                         + bytes(color)
-                        + self._palette[index * 3 + 3 :]
+                        + self._palette[index * mode_len + mode_len :]
                     )
                 else:
                     self._palette += bytes(color)