Originally Posted by
GrumpyDiver
Without getting into details, this is exactly the outcome one would expect. The chlorophyll in grass and other leaves is transparent to IR and certainly in colour IR film, magenta grass and leaves were the norm. The red detect layer in the Foveon sensor would be sensitive to any IR coming through, now that the IR cutoff filter has been removed. Putting an IR/UV filter back on the camera means that you have sort of reverted back to what it was designed to handle, but the IR cutoff frequencies are not the same as the filter that was over the sensor, and some near IR is getting through. Frankly, I'm not quite sure why you would remove the internal IR / UV filter only to replace it with an external one.
The fact that a custom white balance is not happening is a combination of you operating the camera outside of the parameters it was designed to operate under. You might be able to get a bit more correction in post-processing, but I certainly wouldn't expect miracles and you will see knock-on effect; if you increase the green component in the grass, any other green in the image will increase as well.
Nicely said; you have decided to get into IR photography, and you are certainly seeing the results. You might want to consider converting to B&W where all the funky colour effects disappear; you do need a B&W IR filter to do this; I think something like a Wratten 25A was what I used to use (very red colour) with B&W IR film. That will get you white leaves and light gray to white grass, with most other things looking more normal.
You seem to be finding out the hard way, the laws of physics are driving your results. If you understand the underlying technology, you should be able to tweak the results a bit.