GPencil: Add stroke color to Materials Popover

This makes more easy to manage the color, specially for advanced brushes with pinned materials.

Reviewers: @pepeland, @mendio

release/scripts/startup/bl_ui/properties_grease_pencil_common.py

@@ -881,6 +881,22 @@ class GreasePencilMaterialsPanel:
                     row.operator("gpencil.stroke_change_color", text="Assign")
                     row.operator("gpencil.color_select", text="Select").deselect = False
                     row.operator("gpencil.color_select", text="Deselect").deselect = True
+        # stroke color
+            ma = None
+            if is_view3d and brush is not None:
+                gp_settings = brush.gpencil_settings
+                if gp_settings.use_material_pin is False:
+                    ma = ob.material_slots[ob.active_material_index].material
+                else:
+                    ma = gp_settings.material
+
+            if ma is not None and ma.grease_pencil is not None:
+                gpcolor = ma.grease_pencil
+                if gpcolor.stroke_style == 'SOLID' or \
+                    gpcolor.use_stroke_pattern is True or \
+                    gpcolor.use_stroke_texture_mix is True:
+                    row = layout.row()
+                    row.prop(gpcolor, "color", text="Stroke Color")
 
         else:
             space = context.space_data