Why are some materials not showing up in Solid view? I have been having a lot of trouble getting materials to show up, not just in solid view, but at all. I have gone through a large number of versions of this file and thought maybe something got screwed up somewhere along the way, so I deleted all the materials and started rebuilding them. But the problem shows signs of returning, as now one material is visible in solid view, but as I create more they are only visible in material or render view. They are just basic flat colors.

I don't know a lot - I've been switching back and forth between Blender and Cycles Render as I've been building this. Could that be the issue?
When you set a material in the Blender Internal render engine, then it automatically sets the viewport color for you, unlike the Cycles render engine which you have to do manually. When you switch from Blender Internal to Cycles, the Viewport Colors remain.



To set it in Cycles, go to Materials > Settings and change the Viewport Color value: