This tutorial explains a technique I used a lot in grad school to get really fast renderings for studying building forms. Materials are not necessary in the 3-D model which is the case many times early on in the design process.
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KERKYTHEA INFO: This tutorial uses a program call Kerkythea which can be downloaded for free from http://www.kerkythea.net/joomla/ . I have used Kerkythea throughout Grad and undergrad architecture school and is still my rendering program of choice.
A plug-in for Sketchup can also be downloaded from the website. This allows you to export SU model seamlessly into Kerkythea.