diff --git a/pyproject.toml b/pyproject.toml
index a6c57432..85439050 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -8,7 +8,7 @@ description = "set of tools to work with scales, modes, modulations, chord progr
 readme = "README.md"
 requires-python = ">=3.11"
 dependencies = [
-    "colortool>=0.5.3",
+    "colortool>=0.6.0",
     "opseq>=0.1.2",
     "mido>=1.3.0",
     "svg.py",
diff --git a/src/musiclib/svg/isomorphic/base.py b/src/musiclib/svg/isomorphic/base.py
index 874250a6..bd94b0c8 100644
--- a/src/musiclib/svg/isomorphic/base.py
+++ b/src/musiclib/svg/isomorphic/base.py
@@ -257,7 +257,7 @@ def add_key(self, row: float, col: float) -> None:
         polygon = svg.Polygon(
             class_=['polygon-transparent'],
             points=points,  # type: ignore[arg-type]
-            fill=Color.from_rgba_int((0, 0, 0, 0)).css_rgba,
+            fill=Color.from_rgba_int((0, 0, 0, 0)).css_rgb,
         )
         polygon.interval = interval  # type: ignore[attr-defined]
         polygon.abstract_interval = abstract_interval.interval  # type: ignore[attr-defined]