Raise ValueError instead of asserting ImageOps colorize() arguments

Tests/test_imageops.py

@@ -423,6 +423,33 @@ def test_colorize_3color_offset() -> None:
     )
 
 
+def test_colorize_invalid_mode() -> None:
+    with pytest.raises(ValueError, match="mode must be L, not RGB"):
+        ImageOps.colorize(hopper("RGB"), "black", "white")
+
+
+@pytest.mark.parametrize("blackpoint, whitepoint", ((-1, 255), (0, 256), (200, 50)))
+def test_colorize_invalid_points(blackpoint: int, whitepoint: int) -> None:
+    with pytest.raises(ValueError, match="blackpoint and whitepoint must each be"):
+        ImageOps.colorize(
+            hopper("L"), "black", "white", blackpoint=blackpoint, whitepoint=whitepoint
+        )
+
+
+@pytest.mark.parametrize("midpoint", (100, 200))
+def test_colorize_invalid_midpoint(midpoint: int) -> None:
+    with pytest.raises(ValueError, match="midpoint must be between or equal to"):
+        ImageOps.colorize(
+            hopper("L"),
+            "black",
+            "white",
+            mid="blue",
+            blackpoint=125,
+            midpoint=midpoint,
+            whitepoint=175,
+        )
+
+
 def test_exif_transpose() -> None:
     exts = [".jpg"]
     if features.check("webp"):

src/PIL/ImageOps.py

@@ -200,12 +200,18 @@ def colorize(
     :return: An image.
     """
 
-    # Initial asserts
-    assert image.mode == "L"
-    if mid is None:
-        assert 0 <= blackpoint <= whitepoint <= 255
-    else:
-        assert 0 <= blackpoint <= midpoint <= whitepoint <= 255
+    if image.mode != "L":
+        msg = f"mode must be L, not {image.mode}"
+        raise ValueError(msg)
+    if not 0 <= blackpoint <= whitepoint <= 255:
+        msg = (
+            "blackpoint and whitepoint must each be between or equal to 0 and 255, "
+            "with blackpoint less than or equal to whitepoint"
+        )
+        raise ValueError(msg)
+    if mid is not None and not blackpoint <= midpoint <= whitepoint:
+        msg = "midpoint must be between or equal to blackpoint and whitepoint"
+        raise ValueError(msg)
 
     # Define colors from arguments
     rgb_black = cast(Sequence[int], _color(black, "RGB"))