"most developers will produce early versions of a game that has either placeholder art or simple white, grey or orange"

Well well well, I never knew about that side of the game development process. I would have thought that all the art would have been done first then they would make stuff fit, or at least make a basic mock on paper that they know will work, then do the art then fill it in.

Its really interesting to know even vast great scenes with beautifully designed environments and buildings were not based around the art but the art round them. I remember seeing behind the scenes on games where they had the grid style walls and colourless objects, most memorably in Ratchet and Clank. In the game there was a whole secret 'museum' of behind the scenes artefacts and play test items to play with. However even after seeing these I imagined these were just untextured and simplified for testing and that they were just toggling the art on/off as they worked.

I suppose this is so they can know if puzzles/jumps can be completed etc.

this makes an artists work even more interesting with a new level of problem solving they have to achieve. I love stuff like this and which I knew about it earlier. I cant wait to move on to UDK projects and really get to feel like I'm part of a bigger picture.