Rendering with NN Interpolation I have a high resolution texture with only seven colors that I apply to a makehuman model and I want in my rendered result also just those seven colors (plus a black background). 

So far I have tried to to deactivate MIP Map and Interpolation, and different filters with small filter sizes in the Image Sampling panel. However, the result is as depicted in the following image:



Further, I tried to deactivate the Anti-Aliasing in the Render options, but still I do not get the desired result:



You see that the color fades, especially on color borders. The rendered image is rather small (160x120), but also if I increase the dimension, the problem remains.
Deactivating AA and MipMap + Interpolation is the right thing to do. The filters are used with interpolation. The bigger the filter size the smoother the result becomes. But since you don't want any smoothness you should deactivate interpolation. That way you can forget about filters.

Nevertheless Blender seems to sample textures internally with some interpolation.

As a workaround you could use an image manipulation program like GIMP or Photoshop and reduce the colors there. In GIMP you can do it like this: Image > Mode > Indexed.

To automate this task for multiple images you can use BIMP. It is GIMP plugin which is probably already installed. You can find it in File > Batch Image Manipulation...
You can use the filter called "gimp-image-convert-indexed"