Since wireframes are meant to convey design concepts and information architecture, visual clarity is an absolute must. Stakeholders need to be able to see the structure and strategy for the User Interface system without getting distracted by how each element will look in the final design. A wireframe should allow them to examine the what, why and where of screen elements and how they should function.
Focusing on shades and colors when creating wireframes often causes clients to get hung up on how the different screen elements will look. Even if you tell them the aesthetics don’t reflect the visual concepts to be designed later, it’s inevitable that the critiquing of the wireframes will be affected because it’s a natural reaction to what they see. Keeping the visual style of the wireframes clean and simplistic will help clients think about navigational concepts, and not visual ones.