Hello,
this problem has been here with us for a while. Now it seems we have all the info needed to make a statement about this and how to avoid this behavior.
The purple flickering is noticeable while using RenderTargetUsage.DiscardContents for rendering to textures. Neoforce is rendering every control to a texture, so switching between render targets is pretty common. in XNA, every time you switch to new render target, the previous render target gets cleared with the purple color (oficially undefined content). This is not happening for RenderTargetUsage.PreserveContents, so the samples are working just well, while for your application this doesn't have to be the case.
The correct approach how to use Neoforce in these conditions is as follows:
1) let Neoforce draw controls in another render target
2) resolve and save texture from Neoforce render target
3) draw your stuff to back buffer
4) draw Neoforce texture at the end
You can see a sample project of this approach in Neoforce samples folder called RenderTargets or in the new Central application.