Grease Pencil: Channel color for groups

`channel_color` property was created for layers in 03b2fc744e. Now
exposed for layer groups too.

Pull Request: https://projects.blender.org/blender/blender/pulls/135088

scripts/startup/bl_ui/properties_data_grease_pencil.py

@@ -334,6 +334,28 @@ class DATA_PT_grease_pencil_layer_display(LayerDataButtonsPanel, GreasePencil_La
     bl_options = {'DEFAULT_CLOSED'}
 
 
+class DATA_PT_grease_pencil_layer_group_display(Panel):
+    bl_label = "Display"
+    bl_space_type = 'PROPERTIES'
+    bl_region_type = 'WINDOW'
+    bl_context = "data"
+    bl_options = {'DEFAULT_CLOSED'}
+
+    @classmethod
+    def poll(cls, context):
+        grease_pencil = context.grease_pencil
+        return grease_pencil and grease_pencil.layer_groups.active
+
+    def draw(self, context):
+        layout = self.layout
+        layout.use_property_split = True
+
+        grease_pencil = context.grease_pencil
+        group = grease_pencil.layer_groups.active
+
+        layout.prop(group, "channel_color", text="Channel Color")
+
+
 class DATA_PT_grease_pencil_onion_skinning(DataButtonsPanel, Panel):
     bl_label = "Onion Skinning"
 
@@ -494,6 +516,7 @@ classes = (
     DATA_PT_grease_pencil_layer_adjustments,
     DATA_PT_grease_pencil_layer_relations,
     DATA_PT_grease_pencil_layer_display,
+    DATA_PT_grease_pencil_layer_group_display,
     DATA_PT_grease_pencil_onion_skinning,
     DATA_PT_grease_pencil_onion_skinning_custom_colors,
     DATA_PT_grease_pencil_onion_skinning_display,

source/blender/editors/animation/anim_channels_defines.cc

@@ -4055,7 +4055,7 @@ static bAnimChannelType ACF_GPLGROUP = {
     /*channel_role*/ ACHANNEL_ROLE_EXPANDER,
 
     /*get_backdrop_color*/ greasepencil::layer_group_color,
-    /*get_channel_color*/ nullptr,
+    /*get_channel_color*/ greasepencil::layer_channel_color,
     /*draw_backdrop*/ acf_group_backdrop,
     /*get_indent_level*/ acf_generic_indentation_0,
     /*get_offset*/ greasepencil::layer_offset,