Fix python-pillow#6652: Handle translucent color used in RGB ImagePallete

Author: jsbueno

Tests/test_imagepalette.py

@@ -50,6 +50,15 @@ def test_getcolor():
         palette.getcolor("unknown")
 
 
+def test_getcolor_raises_on_incompatible_color():
+    palette = ImagePalette.ImagePalette(mode="RGB")
+    # Opaque RGBA colors should work
+    palette.getcolor((0, 0, 0, 255))
+    assert palette.getcolor((0, 0, 0)) == palette.getcolor((0, 0, 0, 255))
+    with pytest.raises(ValueError):
+        palette.getcolor((0, 0, 0, 128))
+
+
 @pytest.mark.parametrize(
     "index, palette",
     [

src/PIL/ImagePalette.py

@@ -114,9 +114,13 @@ def getcolor(self, color, image=None):
         if self.rawmode:
             raise ValueError("palette contains raw palette data")
         if isinstance(color, tuple):
-            if self.mode == "RGB":
-                if len(color) == 4 and color[3] == 255:
+            if self.mode == "RGB" and len(color) == 4:
+                if color[3] == 255:
                     color = color[:3]
+                else:
+                    raise ValueError(
+                        "RGB ImagePalette can't handle non-opaque RGBA colors"
+                    )
             elif self.mode == "RGBA":
                 if len(color) == 3:
                     color += (255,)