The model I have been working on has been trying to change it's colour all day. Finally I had a crash 'graphics adapter stopped working' and when I re-opened the model it had gone all blue. The top level assembly is all blue but the subassemblies are the proper colours when you open them.
I'm updating my graphics driver which should maybe stop the problem re-ocurring but in the meantime I need to get my top level assembly to display as per the colours in the sub assemblies.
If you can launch Inventor, Go to Tools -> Application options -> Hardware tab -> Change the option to either compatibility view or software graphics. This would help your graphic card to work better with Inventor. But when you change to software graphics, inventor is running on a low graphic setting. So it will be slightly on the slower side.