Do this:
StackHorizontal(StackVertical( \
ConvertToRGB(matrix="bt.601"), \
ConvertToRGB(matrix="pc.601")), \
StackVertical( \
ConvertToRGB(matrix="bt.709"), \
ConvertToRGB(matrix="pc.709")), \
)
One of them is bound to match your expectations, unless you use a totally different colormatrix.
(I don't think any of them will give you 159 or 161 - $99 or $A1 - unless you're converting back to yuv by hand.)
Oh! And make sure you use avisynth 2.5.7. There was a lot of discussion between .6 and .7 about colorbars, and some changes were made to it.
I can throw together a small plugin that just plops YUV values into RGB if you'd like that. Since it probably isn't particularly easy to get it to show in avsp otherwise.