fix: do not share the Expr for text_color and background_color

Signed-off-by: Maarten A. Breddels <maartenbreddels@gmail.com>

ipydatagrid/cellrenderer.py

@@ -6,7 +6,17 @@
 from bqplot import ColorScale, Scale
 from ipywidgets import Color, Widget, widget_serialization
 from py2vega import Variable, py2vega
-from traitlets import Any, Bool, Enum, Float, Instance, Unicode, Union, validate
+from traitlets import (
+    Any,
+    Bool,
+    Enum,
+    Float,
+    Instance,
+    Unicode,
+    Union,
+    default,
+    validate,
+)
 
 from ._frontend import module_name, module_version