That's weird. It did work in wwise 2019.2.5 or 2021.1.0 last week. But I can not modify OverrideColor value now, even in 2019.2.8.
And I also changed the IsVisible value in WObjects.xml.
https://www.audiokinetic.com/qa/6922/waapi-changing-color-not-working-overridecolor-property
But it seems to make the OverrideColor property be available on "All Properties" tab in Sound Property Editor. The error report in 2019.2.8:
2021-01-06T16:37:15 WaapiClientAutobahn (ERROR): ('ApplicationError(error=<ak.wwise.invalid_arguments>, args=[], '
"kwargs={'message': 'Argument property is unknown.', 'details': "
"{'argumentName': 'property ', 'typeUri': "
"'ak.wwise.schema_validation_failed', 'schemaKeyword': "
"'additionalProperties', 'argumentPointer': '/property ', 'procedureUri': "
"'ak.wwise.core.object.setProperty', 'schemaExpect': 'false', "
'\'schemaPointer\': \'\', \'argumentValue\': \'"OverrideColor"\', '
"'schemaDescription': ''}}, enc_algo=None, callee=None, callee_authid=None, "
'callee_authrole=None, forward_for=None)')