diff --git a/colrv1_add_border_to_flags.py b/colrv1_add_border_to_flags.py
new file mode 100644
index 000000000..8893766e5
--- /dev/null
+++ b/colrv1_add_border_to_flags.py
@@ -0,0 +1,132 @@
+"""Utility to add border and soft-light effects to NotoColorEmoji-COLRv1 region flags."""
+import sys
+import subprocess
+from fontTools import ttLib
+from fontTools.ttLib.tables import otTables as ot
+from fontTools.colorLib.builder import LayerListBuilder
+from nototools import font_data
+from flag_glyph_name import flag_code_to_glyph_name
+from map_pua_emoji import get_glyph_name_from_gsub
+
+
+# In the CBDT font, the following flags only get no border:
+# https://github.com/rsheeter/warp/issues/20
+BORDERLESS_FLAGS = (
+    "GF",
+    "MQ",
+    "NP",
+    "PM",
+)
+
+# The two glyphs representing respectively the flag's border and the soft-light
+# effect are expected to be c'mapped to the following PUA codepoint:
+BORDER_CODEPOINT = 0x100000
+SOFT_LIGHT_CODEPOINT = 0x100001
+
+
+def read_makefile_variable(var_name):
+    # see `print-%` command in Makefile
+    value = subprocess.run(
+        ["make", f"print-{var_name}"], capture_output=True, check=True
+    ).stdout.decode("utf-8")
+    return value[len(var_name) + len(" = ") :].strip()
+
+
+def flag_code_to_sequence(flag_code):
+    # I use the existing code to first convert from flag code to glyph name,
+    # and then convert names back to integer codepoints since it already
+    # handles both the region indicators and subdivision tags.
+    name = flag_code_to_glyph_name(flag_code)
+    assert name.startswith("u")
+    return tuple(int(v, 16) for v in name[1:].split("_"))
+
+
+_builder = LayerListBuilder()
+
+
+def _build_paint(source):
+    return _builder.buildPaint(source)
+
+
+def _paint_colr_glyph(glyph_name):
+    return _build_paint({"Format": ot.PaintFormat.PaintColrGlyph, "Glyph": glyph_name})
+
+
+def _paint_composite(source, mode, backdrop):
+    return _build_paint(
+        {
+            "Format": ot.PaintFormat.PaintComposite,
+            "SourcePaint": source,
+            "CompositeMode": mode,
+            "BackdropPaint": backdrop,
+        }
+    )
+
+
+def main():
+    try:
+        input_file, output_file = sys.argv[1:]
+    except ValueError:
+        print("usage: colrv1_add_border_to_flags.py INPUT_FONT OUTPUT_FONT")
+        return 2
+
+    font = ttLib.TTFont(input_file)
+
+    if "COLR" not in font or font["COLR"].version != 1:
+        print("error: missing required COLRv1 table")
+        return 1
+
+    cmap = font_data.get_cmap(font)
+    try:
+        border_name = cmap[BORDER_CODEPOINT]
+        soft_light_name = cmap[SOFT_LIGHT_CODEPOINT]
+    except KeyError as e:
+        print(f"error: missing required PUA codepoint: {hex(e.args[0])}")
+        return 1
+
+    colr_glyphs = {
+        rec.BaseGlyph: rec
+        for rec in font["COLR"].table.BaseGlyphList.BaseGlyphPaintRecord
+    }
+    assert border_name in colr_glyphs
+    assert soft_light_name in colr_glyphs
+
+    regular_flags = []
+    borderless_flags = []
+    for flag_code in read_makefile_variable("SELECTED_FLAGS").split():
+        flag_sequence = flag_code_to_sequence(flag_code)
+        flag_name = get_glyph_name_from_gsub(flag_sequence, font)
+        if flag_name is not None:
+            flag = colr_glyphs[flag_name]
+            if flag_code in BORDERLESS_FLAGS:
+                borderless_flags.append(flag)
+            else:
+                regular_flags.append(flag)
+
+    # For regular flags, first the border is multiplied onto the flag, then
+    # the soft-light effect is applied to the result.
+    for flag in regular_flags:
+        flag.Paint = _paint_composite(
+            source=_paint_colr_glyph(soft_light_name),
+            mode=ot.CompositeMode.SOFT_LIGHT,
+            backdrop=_paint_composite(
+                source=flag.Paint,
+                mode=ot.CompositeMode.MULTIPLY,
+                backdrop=_paint_colr_glyph(border_name),
+            ),
+        )
+    # borderless flags only get the soft-light effect
+    for flag in borderless_flags:
+        flag.Paint = _paint_composite(
+            source=_paint_colr_glyph(soft_light_name),
+            mode=ot.CompositeMode.SOFT_LIGHT,
+            backdrop=flag.Paint,
+        )
+
+    font_data.delete_from_cmap(font, (BORDER_CODEPOINT, SOFT_LIGHT_CODEPOINT))
+
+    font.save(output_file)
+
+
+if __name__ == "__main__":
+    sys.exit(main())