The Designer barfs on this line of code in a Custom View.
The reason is the uppercase 'X' in DefaultValue, it needs to be lower-case. This is inconsistent with the fact that it accepts upper-case for the actual hex value characters.
B4X:
#DesignerProperty: Key: TableColor, DisplayName: Table color, FieldType: Color, DefaultValue: 0XFFF8F8F8, Description: Table and grid background color.