At this point, however, we can begin to look at some complications. There's two areas where I've skipped over some awkward points that get in the way of MVC theory. The first problem area is to deal with setting the color of the variance. This shouldn't really fit into a domain object, as the color by which we display a value isn't part of the domain. The first step in dealing with this is to realize that part of the logic is domain logic. What we are doing here is making a qualitative statement about the variance, which we could term as good (over by more than 5%), bad (under by more than 10%), and normal (the rest). Making that assessment is certainly domain language, mapping that to colors and altering the variance field is view logic. The problem lies in where we put this view logic - it's not part of our standard text field.
This kind of problem was faced by early smalltalkers and they came up with some solutions. The solution I've shown above is the dirty one - compromise some of the purity of the domain in order to make things work. I'll admit to the occasional impure act - but I try not to make a habit of it.
-- http://martinfowler.com/eaaDev/uiArchs.html#ModelViewController