Are UI choices functional or non-functional requirements?

Question

Today we had a discussion at the office about UI elements in the required document.

The question is whether a UI requirement is functional or non-functional.

• "the background must be blue"

Is this a functional or non-functional requirement?

Answer 1

That is a non-functional requirement, it states how a system is to perform, not what it is to perform.

If this clear distinction is too black and white, too simple perhaps, ignore it or provide a better one ! And, why do you care ?

EDIT: prompted by the other answers. Whether the requirement is functional or non-functional is immaterial to its requirementhood -- if it's a requirement you'd better satisfy it in your product.

Answer 2

When looking at the two definitions as given by Wikipedia, I would say neither.

a non-functional requirement is a requirement that specifies criteria that can be used to judge the operation of a system, rather than specific behaviors

a functional requirement defines a function of a software system or its component

The color of the background is not a specific function or behavior of the system and it also can not be used to judge the operation of a system.

I think that these kind of UI requirements do not need to be specified at the beginning of a software design cycle. What if during development you find out that the wrong type of blue was described? These choices are not requirements, but need to be sorted out during UI development (preferably with a user group to judge).