I'd split out type constraint definitions into a separate type library module. Here's a quick example:

use strict;
use warnings;

# thanks.pm is a shim for declaring multiple packages in the same file
+.
# It's on CPAN if you want it. Otherwise, just split them into separat
+e
# files.
no thanks qw(
    MyApp::Types
    MyApp::Printer
    MyApp::Printer::ThreeColour
    MyApp::Printer::FourColour
);

BEGIN {
    package MyApp::Types;
    use Type::Library -base, -declare => qw( ThreeColour FourColour );
    use Type::Utils;
    use Types::Standard -types;
    use Types::ReadOnly -types;
    
    declare ThreeColour, as Locked[
        Dict[
            cyan    => Optional[Num],
            magenta => Optional[Num],
            yellow  => Optional[Num],
        ]
    ], coercion => 1;
    
    # black is referred to as "key" in CYMK printing. presumably becau
+se
    # using a name that abbreviated to "B" would be confused with "blu
+e".
    declare FourColour, as Locked[
        Dict[
            @{ ThreeColour->parent->wrapped->parameters }, # voodoo?
            key => Optional[Num],
        ]
    ], coercion => 1;
};

BEGIN {
    package MyApp::Printer;
    use Moose::Role;
    
    # Let's assume the printer is only capable of printing one pixel.
    has pixel => (is => 'ro');
    
    sub dump {
        require Data::Dumper;
        Data::Dumper::Dumper(@_);
    }
};

BEGIN {
    package MyApp::Printer::ThreeColour;
    
    use MyApp::Types -types;
    
    use Moose;
    with qw(MyApp::Printer);
    
    has '+pixel' => (isa => ThreeColour, coerce => 1);
};

BEGIN {
    package MyApp::Printer::FourColour;
    
    use MyApp::Types -types;
    
    use Moose;
    with qw(MyApp::Printer);
    
    has '+pixel' => (isa => FourColour, coerce => 1);
};

my $printer = 'MyApp::Printer::FourColour'->new(
    pixel => { cyan => 9.4, magenta => 7, yellow => 4 },
);

print($printer->dump);

$printer->pixel->{key} = 9; # ok

print($printer->dump);

$printer->pixel->{black} = 6; # dies
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Very informative and I appreciate you taking the time to explain... many thanks!