diff --git a/Tests/test_image_access.py b/Tests/test_image_access.py
index bb09a7708..a000cb64c 100644
--- a/Tests/test_image_access.py
+++ b/Tests/test_image_access.py
@@ -345,13 +345,14 @@ class TestCffi(AccessTest):
 
     @pytest.mark.parametrize("mode", ("P", "PA"))
     def test_p_putpixel_rgb_rgba(self, mode):
-        for color in [(255, 0, 0), (255, 0, 0, 127)]:
+        for color in ((255, 0, 0), (255, 0, 0, 127 if mode == "PA" else 255)):
             im = Image.new(mode, (1, 1))
             access = PyAccess.new(im, False)
             access.putpixel((0, 0), color)
 
-            alpha = color[3] if len(color) == 4 and mode == "PA" else 255
-            assert im.convert("RGBA").getpixel((0, 0)) == (255, 0, 0, alpha)
+            if len(color) == 3:
+                color += (255,)
+            assert im.convert("RGBA").getpixel((0, 0)) == color
 
 
 class TestImagePutPixelError(AccessTest):
diff --git a/Tests/test_imagepalette.py b/Tests/test_imagepalette.py
index 475d249ed..5bda28117 100644
--- a/Tests/test_imagepalette.py
+++ b/Tests/test_imagepalette.py
@@ -50,6 +50,16 @@ def test_getcolor():
         palette.getcolor("unknown")
 
 
+def test_getcolor_rgba_color_rgb_palette():
+    palette = ImagePalette.ImagePalette("RGB")
+
+    # Opaque RGBA colors are converted
+    assert palette.getcolor((0, 0, 0, 255)) == palette.getcolor((0, 0, 0))
+
+    with pytest.raises(ValueError):
+        palette.getcolor((0, 0, 0, 128))
+
+
 @pytest.mark.parametrize(
     "index, palette",
     [
diff --git a/src/PIL/ImagePalette.py b/src/PIL/ImagePalette.py
index b73b2cd9d..fe76c86f4 100644
--- a/src/PIL/ImagePalette.py
+++ b/src/PIL/ImagePalette.py
@@ -115,7 +115,11 @@ class ImagePalette:
             raise ValueError("palette contains raw palette data")
         if isinstance(color, tuple):
             if self.mode == "RGB":
-                if len(color) == 4 and color[3] == 255:
+                if len(color) == 4:
+                    if color[3] != 255:
+                        raise ValueError(
+                            "cannot add non-opaque RGBA color to RGB palette"
+                        )
                     color = color[:3]
             elif self.mode == "RGBA":
                 if len(color) == 3: