Hi,
My advice to you would be to link to the separate top level categories if it is useful to the website user. Build the website for the user, not for search engines, and the rankings will come.
E.g on the green widgets page you could have a clear funnel to each type of green widget and then some text such as "Prefer red widgets? We've got a great range of red widgets available for purchase." Again, I would only use this if it was useful to the website user.
Another example might be if green widget 2 is exactly the same as red widget 2 except for the colour. You'd obviously want to link these 2 as they are related items and the user might want to visit the other colour widget.
Thanks