I revisited the github issue and decided to take a look at what ExtUtils::Installed has to say about Color::Rgb:

% perl -MExtUtils::Installed -le '@_=ExtUtils::Installed->new->modules;@_=grep/color::rgb/i,@_;print for@_'
Color::RGB
Color::RGB::Util

What about the packlist?

% perl -MExtUtils::Installed -le 'print ExtUtils::Installed->new->packlist("Color::RGB")->packlist_file'
/perl5/perlbrew/perls/perl-5.42.0/lib/site_perl/5.42.0/darwin-2level/auto/Color/RGB/.packlist

Inside the packlist?

% cat /perl5/perlbrew/perls/perl-5.42.0/lib/site_perl/5.42.0/darwin-2level/auto/Color/RGB/.packlist
/perl5/perlbrew/perls/perl-5.42.0/lib/site_perl/5.42.0/Color/Rgb.pm
/perl5/perlbrew/perls/perl-5.42.0/man/man3/Color::Rgb.3

The source of this issue still eludes me but I don't think perlbrew is the problem anymore. The output of Module::Metadata shows nothing unusual, module is reported as Color::Rgb.