How do you want the Color defined?
The simplest is just to inject a Color instance into the fixture before you start creating objects:
This will reuse the same Color instances for all created objects.
Alternatively, you can use the Register method to specify a function that creates new instances every time it's invoked. That might look something like this:
Where CreateColor is a method with this signature:
You can also use a code block (lambda expression) if you prefer.
Personally, I'd implement the desired behavior in a custom ISpecimenBuilder - it's actually quite easy.