The "--no-color" [1] parameter in "appstream-validate --help" uses a wrong description.
Instead "Don't show colored output" [2] should be used.

[1] https://github.com/ximion/appstream/blob/master/tools/appstream-validate.c#L160
[2] https://github.com/ximion/appstream/blob/master/tools/appstream-index.c#L517