That's a lot of work for the calling code. I'm not a Moose expert either but I'd use a BUILDARGS method, that's significantly shorter and cleaner already:

sub BUILDARGS {
    my ($class, %args) = @_;
    defined $args{colour} and '' ne $args{colour} or $args{colour} = '
+#123';                                      
    return \%args;
}
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Like this you only have to write the whole stuff once instead of once for every ->new